On the good news side: I think I've found the main entrance to the house. Not a single squeak has been heard inside since.

On the not so good news side: too many alternate targets. We're now replacing the vehicle air cleaners annually; my car also used to have covers over the wheel wells, sealing the engine compartment; motorcycle covers are well ventilated; ... So far, no brake fluid detected on the deck :-/

Sadly, we have something called fisher cats (actually a large weasel) prowling the outdoors. Our last MrOWWwwwrrrr! didn't make it past his first birthday :-(