I don't notice when people flirt with me and I don't notice when I flirt with other people. :( It always has to be pointed out to me. Sometimes I'll realize in retrospect (ie, hours, days, weeks or even months later) that someone was flirting with me and it went right over my head and be kind of embarrassed about my inability to detect it.
Flirting is annoying when it's terribly obvious to me that you are flirting (be it with me or anyone) and it really bothers me once I've noticed it, for some reason. I don't find it creepy unless, like some people have said, it's in an inappropriate location/time or becomes sexualized or when it's clear the other party isn't interested and the original person keeps pursuing it anyway.