It could be anything, something weird at night, a sound, a light or a cryptid.
There is a theatre attached to our house. It has only existed since 1991, before that it was a single story machine workshop, and before that - all manner of other things. There is a large Theatre Pipe Organ in it, and we host midweek groups who come down on their special outings (WI, Probus Club etc). One day we had one of these groups in, and as usual when they left we went around the building checking the toilets etc to make sure everyone was out. Most of the time people arrived by coach, but sometimes if the group was closer to us, and a smaller size, they came in cars.
This day we checked everyone was out, and as usual quite exhausted - we locked up the building and went through to the house for a smoke and a coffee before heading back through to tidy away the plates and bowls and start collating the left over scones, butter, jam, cakes etc so store in a fridge and pick our way through over the next several days.
On returning back to the foyer - the fire extinguisher which sits on a bracket on the wall, was sitting upright in the middle of the floor. It couldn’t have fallen off the bracket, because it would be laying in it’s side near the wall in that case. It was sitting upright - a good 3-4 metres away from the wall. There is simply no way it could naturally have ended up where it was - without someone physically putting it there. But the building was locked, and we would definitely have noticed if it was there when we locked up the building and left, because we would have had to walk around it to leave the building.