You DEFINATELY need more litterboxes. I have 7 cats, and 7 extra giant litterboxes (some of my cats are big).

Yeah they don't look exactly like framed artwork for decor but they get the job done. I have 3 in my front porch area which is actually the front part of my livingroom. I have 2 in a spare bedroom upstairs, and 2 in my own bedroom. I figure I'd rather look at, and have my company look at a whole bunch of litterboxes, than have my house smell up and be ruined with cat urine.

Get a few more boxes and it should help at least a little until your video cam catches the culprits so you can have them vet checked for UTI's.

Good luck.