Sorry, is anyone sitting here?
What it really means
The polite way to ask for a free seat on a train, in a pub or cafe. It really means "may I sit here?". The usual answer is "no, go ahead". Taking a clearly reserved seat without asking is frowned upon.
Context, tips & common mistakes
"Is anyone sitting here?" (or 'is this seat taken?' / 'is this free?') is the required ritual before taking an adjacent seat in a pub, train or waiting room — even when the seat is obviously empty, because the question is really 'may I enter your space?'. The answers: 'no, go ahead' releases the seat; 'sorry, my friend's there' reserves it, no offence taken either way.
The bag-on-seat variant: a bag often means 'saving it', and the polite challenge is the same question, at which the owner will almost always move it with a 'sorry!'. On busy trains you may add 'do you mind if I…?' while gesturing. Skipping the ritual and simply sitting is not a crime, but the two-second question buys guaranteed goodwill — and in a pub, sitting at strangers' tables without it genuinely raises eyebrows.