Can you buy some sort of a video recorder you could set up?
I opened our mud room door to go into the garage one night and there was a possum sitting on the top step - he ran off though, seemed more nervous of me than I was of him
Can you fit a walk in trap over the opening?
It could be a whole herd of mice (not sure what you call a group of mice) Your kitchen could be party central for the local mouse population - or rats!!!
Squirrels are another possibility, my dogs ignore them for some reason and my father in law woke up one night to find one sitting on his bed - it had come in through a small top window that was open a crack - they are really smart and very agile, can climb pretty much anywhere