The little buggers just look for somewhere warm and dry. No fish oil required.
A couple years back, I got an engine light on our Tacoma on our way south on a big trip. After fighting with our new GPS which led us to Mr. Toyota's home on the far side of Prince George from the automotive dealer, we just made it in before they closed. The mechanic was actually headed out the door when we arrived. He stayed to help us out.
The light was from a contaminated oxygen sensor on the intake air. Contaminated because a squirrel had found its way into the intake air filter, chewing through the filter to get in there of course. Once in there and deciding the space was rather nice, it filled the space with nesting debris, which got sucked past the sensor.
I have a relatively low opinion of rodents...