Police discovered fragments of a "silent conversation" writing in a document on Dart's laptop and then deleting the text.
Counter-terrorism teams also believe that the pair used the same tactic walking down the street with a mobile phone.
Police discovered fragments of text on Dart's laptop that revealed that the pair had used the computer to have a "silent conversation" to avoid possible surveillance bugs.
They would open a Word document and take it in turns to type, before deleting the text and mistakenly assumed that none of it would be stored on the machine.
However forensic experts were able to plough through 2,000 pages of computer code to decipher fragments of what was said.
These included Mahmood making a reference to Wootton Bassett and then adding "if it comes down to it it's that or even just to deal with a few MI5 MI6 heads".
Today the sentencing process, which could take until tomorrow, began at the Old Bailey.
Prosecutor Jonathan Laidlaw QC told the court:
"The method employed as the police, with the help of computer experts, would subsequently discover, involved Dart and Mahmood sitting together at a computer and opening a Word document on the computer to conduct what in effect was a silent conversation.
"Having had that discussion by typing into the document, the document was then deleted by one or other of the defendants, without having been saved and as far as the defendants were concerned the document would therefore be destroyed forever.
"They plainly were under the misapprehension that the text once deleted could never be recovered."
Three men, including a white Muslim convert, are due to be sentenced after pleading guilty to plotting a terror attack in the former military repatriation town of Wootton Bassett.
Richard Dart and his co-conspirators Jahangir Alom and Imran Mahmood admitted to the plot last month. Dart and Alom travelled to Pakistan to try to get terrorist training, and took advice from Mahmood who had already visited the country.
Dart, a former BBC security guard, also discussed bomb making and potential targets with Mahmood, which included the town of Wootton Bassett.