Video report by ITV News Correspondent Dan Rivers
This is their account as a night patrol led them to a group hidden on the northern coastline:
The summer sun had finally melted into the horizon over the beach, bathing the sky primrose and more dramatic by the rags of scattered grey cloud.
We were led into the dunes by another group of gendarmes, led by Major Marie-Laure Pezant, who are one of 15 to 20 foot patrols scouring the French coast each night.
There are currently between 1,000 to 1,500 migrants and refugees along the north of France according to Fabien Sudry, the prefect (top political official) for the Pas-de-Calais region.
Around 300 are in Calais, 800 to 1,000 are in Dunkirk and around 200 are in Caen.
Watch Dan Rivers discuss his analysis with Tom Bradby
When the Calais "Jungle" was at its peak in 2015, Sudry said between 8,000 and 10,000 migrants lived there.
There were 297 crossings in 2018 according to the UK Home Affairs Select Committee, although almost all of those took place from October.
The French Prefecture of the Pas-de-Calais region told ITV News 453 migrants had been intercepted since the beginning of 2019, while 678 migrants have made it across.
He said eight smuggling networks have been exposed and broken up during that time.
On Monday, a man who organised cross-Channel people smuggling attempts was jailed for three years and four months.
Sarbast Mohammed Hama, 31, of no fixed UK address, pleaded guilty to assisting unlawful immigration at Lewes Crown Court.
French Prefecture of the Pas-de-Calais region Fabien Sudry told ITV News that while the majority of traffickers are Kurdish or Iranian, some of them are British. He added that UK taxpayers' money is being used to help equip French authorities in their work to try and stop migrants from crossing the Channel.
Two gendarmes manned a checkpoint nearby, pulling over vehicles randomly, checking the driver’s papers and searching the boot for any sign that they may be carrying out or assisting an illegal crossing.
We asked if they have ever found what they were looking for at any of these checkpoints?
"Rarely. A couple of months ago we pulled over a Ukrainian driving a van with a packed dinghy and life jackets," said Major Pezant, the patrol leader.
"It’s the only time we found something suspicious in this area but he wasn’t doing anything illegal so we let him go," Pezant said.
"The main purpose of the checkpoints is to deter anyone planning a crossing at the main access points to the beaches and to reassure the public."
Had they ever found migrants attempting to cross from this location?
"Again, it’s rare," Pezant said.
More tend to launch from around Wimereux, about 20 miles west of Calais.
We could just make out the outlines of four gendarmes heading west on patrol across an otherwise deserted beach. We went east with four others.
A helicopter hovered around us, providing “air surveillance support” using a thermal imaging camera and spotlight.
The gendarmes wore stab vests as a precaution but were relaxed and almost casual.
After all, sightings of migrants were not only uncommon here but they had never had a violent encounter with any of them.
While their crossings may be illegal, the crime is altogether different from that of more conventional criminals.
They are not murderers, burglars or the human traffickers that sometimes profit from their desperation to find a better life.
They are a combination of refugees and economic migrants fleeing persecution and poverty, prepared to endure great hardship and risk to come to the UK. Most are well-educated.
Lieutenant Colonel Jean-Luc Pereau, one of those on the patrol, said "we must do our job but I do have sympathy for [the migrants] because they are exploited by smugglers who do not care about them".
As we walked briskly across the sand, the barely distinguishable figure of a man was spotted.
Two of the gendarmes used their night vision binoculars and thought this could be a smuggler or scout checking to see if the coast is clear ahead of a launch.
There was a moment of tension as he approaches, running with purpose.
A second later, there were smiles and relief. It was a jogger in his 30s, finishing an evening run along this pristine stretch of beach.
He told the gendarmes he hadn’t seen anything suspicious but would contact them if he did.
The gendarmes took us off the rippled sand and into the thicket-covered dunes.
They found discarded clothes they think may have been left by migrants intending to cross - jeans, a jacket and shoes - it was a glimpse but hardly conclusive.
We were then led through the dunes to an old World War Two German bunker, hidden from the beach in a tangle of brambles and scrub.
This we were told was a favourite hideout for the migrants who shelter here before attempting the 21 mile journey by dinghy to the UK.
This night, though, there was no sign of anyone here.
But then Major Pezant’s radio crackled.
I didn't catch every word from the other patrol several hundred yards down the beach but the message was clear.
They had found six Iranian men hiding in the bush and suspected they were preparing to cross to the UK by boat in the night.
The gendarmes hurried out of the bunker to join their colleagues and found the six men, mostly in their twenties, lined up and surrounded by the gendarmes, in a pool of light cast by their powerful torches.
They looked crest-fallen and worried.
We knew from months of work on this story the vast sums of money they have probably paid to get to France from Iran and the thousands more they may have handed over to smugglers who will have promised them a boat and engine.
It was not clear what had happened in this instance; whether they were still waiting for the boat or if it was hidden somewhere further along the beach.
A closer search of the area where they were found unearthed a rucksack with waterproof covers for their shoes and bodies.
One of the men, Ali, spoke a few words of English and told us he has been in Calais for a week, after a journey overland through Turkey and eastern Europe to France.
This night was the climax of that journey to a better life and it was clear he was bitterly disappointed.
One of the young men was well-dressed in a suit jacket and smart brogues.
Often those fleeing Iran are intellectuals from wealthy families who can afford to send their children to the west.
They are often persecuted minorities - Christians or homosexuals - whose lives are a misery in the hardline Islamic Republic.
The men have official documents showing they have already been charged with being in France illegally.
Two of them were given 30 days to leave France and there were still 10 days before their deadline expired, when in theory they would be deported back to Iran.
In practice though France's concern for their human rights trumps the deportation order and they were all released within 24 hours of their arrest.
The six Iranian men may have been about to carry out an illegal crossing but they could not be deported to a hardline country like Iran, where they would be at risk of torture or death.
So they were caught, questioned, processed and released, free to try their journey again within a few days.
On the same night at nearby Wimereux another 20 Iranians were captured by gendarmes patrols.
They may or may not get their money back from the smugglers, who are undoubtedly the winners in this perverse game of cat and mouse on the beaches of Calais.
The gendarmes' task may seem futile.
"We must do our job," Lieutenant Colonel Pereau said. "And you must imagine how much worse and chaotic it would be if we did not."
Following our night on patrol, a Home Office spokesperson told ITV News: "No one should put their own lives, or the lives of their children, at risk by attempting to cross the Channel in a small boat.
"It is an issue that can only be addressed through collaborative work, which both the UK and France are committed to.
"Since the Home Secretary declared a major incident in December, two cutters have returned to UK waters from overseas operations, we have agreed a joint action plan with France and increased activity out of the Joint Coordination and Information Centre in Calais. Last month, the Home Secretary and the French Interior Minister, Christophe Castaner, agreed to continue to explore options to reinforce the efforts already being made.
"It is an established principle that those in need of protection should claim asylum in the first safe country they reach and since January more than 40 people who arrived illegally in the UK in small boats have been returned to Europe."