Mr Bates was working as a civilian adviser to the police in Afghanistan when he spotted a medal bearing the Duke of Lancaster's Regiment logo at a bazaar.
The Regiment set about tracking down its owner Mick France via its network of ex-soldiers - and by using Facebook.
Supt Bates said: "I regularly trawl the local bazaars in Kabul where India General Service Medals are not uncommon and, while doing so, one of my contacts showed me this medal.
"It was engraved with Mick's name and unit and obviously had a story to tell. So, having bartered the price down, I purchased it and contacted the Duke of Lancaster's Regimental Museum."
Lance Corporal France said: He said: "I was amazed that my medal had been found in a bazaar in Afghanistan - a place that I never went with my regiment.
"I have been to Northern Ireland and Bosnia, but how it turned up in Afghanistan I will never know.
"I'm so grateful and proud. It's absolutely fantastic to get it back. I probably last saw it in Berlin and can only assume that someone borrowed it to wear and never returned it.
"I've had to apologise to my mum - all my old kit is stored at her house and for years I've accused her of losing my medal!"
Mr France was presented with his Northern Ireland service medal for a second time by Mr Bates during a ceremony at the Regiment's headquarters at Fulwood Barracks in Preston.