Thanks to everyone.
So the sensors need to be accessible, but they'll probably be good for 10 years before corrosion is an issue (depending, of course, on environment).
Monk - that position is perfect, below the vanity. Those sensors aren't small, they're tiny!
I hadn't considered a mop setting it off. Maybe some kind of perforated enclosure? Modded plastic circuit project box, with slats extending to floor? I'll take a look at Mouser and Radio Shack. If it's behind the toilet, and painted, attached to the baseboard, the box shouldn't be too bad.
I'll [try to] run the 22/4 to the area behind the MBa toilet as soon as it arrives - hopefully before the painter is all done, with other parts of the house. I hope there's no fire stop...
Edit - Mouser has a tremendous selection of 'project boxes'.