Dame Stella, 77, who is now a bestselling thriller writer, is said to have left the computer on a luggage trolley from where it was taken. It also contained book research. She was said to be "very upset".
The theft of the laptop has been reported to the Met's SO15 Counter Terrorism Command because of security concerns, according to a newspaper.
A source told The Sun: "Dame Stella seems to have forgotten the tricks of her tradecraft since leaving MI5.
"CCTV shows the laptop in the upper compartment of her trolley as she left the terminal building at Heathrow last week.
"It's possible it was taken while her back was turned. But it seems more likely that she left the computer on the trolley and somebody took it after she was driven away.
"There were apparently some contact details of old colleagues."
Dame Stella, the first woman to head MI5 when she took charge in 1992, was also the first head of the service to be identified publicly when she posed for cameras at a brochure launch.
She stood down in 1996 and her memoirs were published in 2001. Since then she has written a series of thrillers about a female intelligence officer.
Dame Stella was not available for comment last night, The Sun reported. Scotland Yard confirmed that the laptop was reported missing last Tuesday.
