Normally I would have chatted back in a more enthusiastically friendly manner, but I had a vague feeling that someone had told me something was suspect about this particular person, so while polite, I made rather more of an effort to move on quickly without engaging than I would do normally.
Only about five minutes later did it occur to me that my vague feelings of distrust and suspicion had their root in the fact that he bore a noticeable resemblance in appearance and clothing to Owen Harper, from Torchwood. Whoops.
People who tell you to 'go with your gut instinct' presumably don't suffer from this kind of problem. My instincts are constantly swayed by being bathed in a sea of suspicious and dubious characters and improbable scenarios from around the galaxy. They rarely seem to have any validity.