Well, that's a bit of a puzzle, ain't it? If you're seeing Bitcoins in your wallet that you don't remember acquiring, there could be several explanations.

Maybe someone sent them to you as a gift or payment. Or perhaps you did some freelance work and got paid in Bitcoin. Could be that you bought them ages ago and just forgot about it.

But if none of those ring a bell, I'd say double-check everything. Make sure the Bitcoins are really there and not just an error or glitch in the wallet app. And most importantly - make sure nobody else has access to your wallet!