As with any scientific inquiry, an observation led to a hypothesis - we have mice. To evaluate the hypothesis, we needed evidence. The most trusted evidence to support or refute a hypothesis is always the most direct. Therefore, the ideal finding would be - a mouse.
When direct evidence is not available other outcomes or markers may be substituted. In this case markers might include clear evidence of mice a) eating or b) pooping. My 'science geek' status affords me some amount of confidence and training in mouse eating and pooping.
Which is why I went on a rampage in the kitchen. Every box of crackers, bag of wheat, and can of coffee was thoroughly inspected. Not a single gnaw mark. I crawled around and inspected the corners of the kitchen floor at close range with flashlight. Not a poo to be found. I reorganized the recycling and cleaned off the breakfast table. Nothing.
I was ready to discard the hypothesis and begin on another - tarantula.
DH, in his wisdom, suggested the introduction of a more elegant observational design. He poured a light dusting of flour across a tight bottleneck between the kitchen and living areas of the apartment. On the second morning our trap captured the proof we feared - several tiny paw prints close to the wall. Housemates. The tiny fury squeaky kind. Great.