Hmm. Some of the responses I've seen seem to rely on the fact that people assume operators don't use common sense.
If someone was qued at my till, and someone arrived after them and all of a sudden they were like "Oh, hey! How are you, haven't seen you in ages, hows ____?", then I would assume that they've happened to bump into each other in my que. It's obvious when people are together and when they bumb into each other, most of the time.
It's like, we do tend to know when you were part of a group and now you're not. The incient I mentionned earlier, with the young lady buying alcohol for her friends? If my manager hadn't have been aware of it, I wouldn't have been. She did not arrive with her friends at my till, and I hadn't seen them in the shop. But a member of the Wines and Spirits had alerted our supervisor and it had gotton through that way. (Next time you go into Morrisons, or any supermarket that operates group ID policies, look around the Wines and spirits. There's normally at least one or two members of staff who look like they're stacking shelves. They're probably not, they're just fiddling with things on shelves while they watch what everyone else is doing.)