I am in Galleries First at T5 and I have myself 'swiped' a couple of small cans of diet coke for later.
After seeing this thread I have positioned myself between the coffee/tea station and beer/soft drinks station in the buffet area.
I would say that whilst I have been discreetly observing for the last 15 minutes (it is reasonably busy) at least 5 people have also popped soft drinks, beer or biscuits/cakes into their bags.

I'll let you know if I get out without being rumbled.