I hate it when I find someone in my seat when flying and when this happens I am not so sure what to say to the seat-thief. It is a really uncomfortable and annoying situation, especially as it happened recently to me, that the person who has taken the seat has lots of hand luggage, coats and so on, all spread out over the row of seats. What do you do in this situation?
Perhaps it was me who you were sitting to, hahaha. Joking aside, I am a bit of a disorganized flyer and it has happened to me that I ended up in the wrong seat. This is simply a question of clumsiness I am not trying to be rude or anything. The bit about the hundreds of things all spread out also applies to me I am afraid. Anyway, the thing to say to the person is clearly just a polite Excuse me, I thought that was my seat, 7a? and then you will compare boarding passes after which the person will move to their own seat. It is probably more awkward for them than for you.