I'm not 100% positive which one it is, but I think it is the baby, Pablo.  He has peed in my daughter's bathtub 3 times now!!  The littler box is in her bathroom.  Pablo is about 4 1/2 months old now and is not fixed yet.  We were away for Christmas and my brother stopped by 1 or 2 times a day to give them food and water and clean the box.  He didn't actually see it happen, but while he was cleaning the box, he said he heard Pablo in the tub trying to "cover" something and he opened the shower curtain to find his liquid present.  I found pee in the tub again the day after we got back, and then again the other day.  I sprayed it really good with Lysol bathroom cleaner and hosed it off with the water as hot as it goes.  Our other cat is 8 months and is fixed, he has never peed on anything (as far as I'm aware).  I googled it and it says this can be a sign of UT problems... but this is really only common in neutered males, which Pablo is not.  Has anyone else dealt with this?  Was it a sign of a bigger problem?

My old cat used to pee on the tile right in FRONT of the tub. We could not get her to stop doing it for over a year, I just was glad it was on tile that I could clean up and not on the carpet! The vet suggested that it was behavioral and gave us medication, but all that did was wipe her out. Literally, she wouldn't move from the same spot all day except to eat, she was so lethargic. So we stopped doing that. I finally kind of figured out around the time she would go and then watch her and when she started going toward the bathroom, I'd put her IN her litter box. Still not really sure what the issue was, but after about 16 months, she completely stopped doing it.

He might have been acting out since you left him. My female cat got mad once and left a nice brown present in the shower. She hasn't done it since and my only conclusion from the situation and reading online is that she was mad.

I would get him to the vet asap inappropriate urination can signal a health issue such as crystals or an infection. He could be marking but if he was neutered as a kitten then that shouldn't be an issue.

Allie have you had any luck? My female cat has just started doing this, she refuses to go in her liter box even thought it is literally brand new.. I guess she hates the new liter I bought? Ugh, its always something!

He's a young unneutered male, since he's reaching puberty for cats he's beginning to start marking behaviors. My advice is get him neutered asap. It's common for a young cat, especially unneutered males, young cats, and cats who are stressed to inappropriately urinate outside the box.
It could be the litter, it could be that he doesn't want to use the other cats box (they get possessive sometimes) or it could just be he's marking. It's not acting out, they don't think spitefully :) Keep in mind when you do neuter him, marking may persist for a couple months but neutering him young will help prevent marking from continuing into adulthood.

Hope this helps, I run the cattery at an animal shelter :)