The other thing that seems to make this unlikely is that I had a back and forth with the owner where we established that neither of us knew whose sign it was or who was moving it, and then a month later I asked again if she knows, and if not, whether I could take it away. I would expect she'd have asked her tenants about it, and also made clear to them (again) that she has rented out the parking spots.

To be honest, I count less and less on proactive communication from other people - it's also entirely possible that she scratched her head, shrugged and forgot about it.