In that pot it can only become a bonsai, but I will probably rip it out before it comes to that and the animal doesn't retrieve it's food until next year, because the roots could come out through the pots bottom drain holes and cause damage.

Yeah, there are, but I have only ever seen them on and between trees, never on houses. Would be nice to have them visit during the day, when my shutters are rolled up so I can admire them.

The hole is also very small and I had blackbirds dig in the pot next to it before, so I also suspect birds. Crows are known to leave gifts and acorns are among the possible choices.

If it was a squirrel, it may come back and search it in a few months.

Thanks for the info. We don't get blue jays here in EU, but lots of sparrows and the eurasian jay known locally as the Eichelhäher (Acorn Jay). The acorn seems too big for regular sparrows beaks, but not sure. If it's an eurasian jay I probably won't see them, because they are very shy here. I only ever saw them from very far, hiding on trees when I heard their warning signal.