You could of course say "is someone sitting there, who has momentarily vacated the seat in order to buy a drink at the bar or visit the toilet?" But it's a bit of a mouthful. It would perhaps be more correct to ask "is this seat taken?" but it sounds a bit poncey. I think most of us would opt for "is anyone sitting there?"

Incidentally Spot, proper Finnann Haddie is not yellow. The yellow stuff is sprayed with artificial dye and is generally inferior in quality to the proper undyed product, which is a pale cream colour, and as has been mentioned several times, absolutely delicious.