TIFU By accepting a house sitting gig

l live in New England and I am house-sitting. Maybe pet sitting is a better description. Either way, I'm supposed to basically live at this house for two weeks. A handful of adorable cats and dogs. Easy money!

Except...

The light was on upstairs after I came back from the store. I did not turn it on. I am certain I did not turn it on. It was not on when I slept last night. I have not gone to that floor more than once because the cats don't go there, there are no litter boxes to scoop, and there are no open doors the cats could get into. I thought maybe it had been on the whole time, but then the light was turned off when I checked later on today.

They didn't say anyone would be staying with me. If someone else was here, why would they need a sitter?

I checked the rooms but found no one. Nothing else has really indicated another person is here. No food missing, no cooking. Some noises like someone walking around or like plumbing is running somewhere, but it's an old house.

I haven't seen another person. But how else would a light turn on and then back off again?

This is not a smart home. It's old and falling apart.

I have to tell myself they're on a timer, but it's freaking me out. The cats and dogs seem calm. Surely if the pets are calm, there's no intruder. The homeowners are unreachable.

I can't leave the pets. I agreed to watch them and they're very picky about their litter boxes (will poop in the sink if they aren't scooped twice a day.) also, I need to replace my windshield and so a house sitting gig will help me save for that.

TLDR; TIFU by agreeing to housesit and potentially rooming with an intruder. I can't do anything about it because if I leave, the pets would be on their own.

ETA: The homeowners are unreachable, and nothing else in the house operates on a timer. The light is on the ceiling, and both the light and the switch are identical to the rest of the lights and switches in the house.

Update!! Got curious. Took out the bulb. Not a smart bulb. Relatively new looking, no soot in the bulb. Flipped the switch and the light turned on.

Second update: Checked the attic! No one was there. Kinda small for anyone to live there. Family is relatively new to the area. Fixture is not a smart fixture. Dogs are small. Have not heard any additional noises tonight. Doors are locked now, no one is in any room of the house. Going to bed under the assumption a cat flipped the switch, but leaving hot cocoa out for the ghosts.