The Pianist

When you play the Re5 DLC Lost in Nightmares (which is the flashback mission where Jill and Wesker end up falling through the window) you can at one point hear someone playing the piano which you have played before to open a hidden door. If you go to the room the music stops before you get there and there's no-one, even though it's a dead end.

It's my theory that it's Wesker there who uses the piano to open some other hidden passage and leaves before Chris and Jill get there.

Even if it wasn't him, I'm sure Wesker can play the piano, considering there are several doors in Umbrella facilities that require that skill to open, and learning something like that would be easy for him.