We’ve been having trouble with rodents (I assume squirrels) in the crawl spaces on our second floor. I decided to stick a PIR sensor on the door to see if I could catch any movement, and sure enough, a bunch of movement around 1 AM this morning. (Correction, it was a possum, which makes sense in hindsight as they’re nocturnal).
As long as the sensor only picks up stuff that’s rat-size or larger then it’s fine. If there are something “large” down there, it can be a hazard to the house foundation. I’d say it’s better to know. 😉