Check your carbon monoxide detectors. SERIOUSLY!

Just had to pick the second one! You really left me no choice!

Do you live in a place with fracking? Have you ever been known to sleep walk? Are you taking Ambien?

This is a trick I had learned when I had an apartment that did not allow pets. Putting aluminum foil on the windowsills is supposed to deter the kitty from going into the window.

I'd be looking for a live trap to set up. A low tech method of surveillance is to spread floor on the counter and the floor around it. I'm sure you will find some kind of animal tracks in it.

Yup set up a camera. No other way to find out. Good luck.

Something might have came in some way. I had field mice come in one time but when I discovered them they left almost the same way they had came in. It was very strange.

May be set up a camera to record what's happening in the kitchen.

Once I was at work alone. I left my phone in the office and I was in the kitchen for awhile at the opposite end of the building. I heard music and went to the office to find my phone playing a song from my music library. My phone was in lock mode, my music app was not open, and it was a song I hadn't listened to in months. And that had never happened before or since.