Introduction going well but now there's marking

[carpdirector]

I've posted a few times about the introduction of a young stray, Charlie, to our resident cats, Simon and Blue. It seems like things are going pretty well. Blue, the alpha cat, often bullies Charlie but he will allow him to nap on the bed with him, sometimes pretty close. Simon has stopped hissing at Charlie, except when Charlie surprises him. Growling has stopped and there is no fighting. Charlie seems to know his place and just wants someone to play with.

However, someone has been peeing in various spots around the house. Today my son saw Simon pee on a spot of carpet where Charlie had missed the litterbox (he was too close to the edge). Apparently I hadn't cleaned the spot well enough. I use an enzyme cleaner to get the smell out, and today I got a black light to help find spots that need better cleaning. What else can I do? Will this eventually stop on its own as Simon gets more comfortable with Charlie?

 

[mani]

I'd clean really thoroughly as you have done, and maybe put something over the spot?

This is an excellent article on all things litterboxy:

 [article="32366"]How To Solve Litterbox Problems In Cats The Ultimate Guide​[/article]