Marking scratch pad?

I have 3 male cats - all neutered (at 6 months). The oldest is 3 and the younger 2 (brothers) are about a year old. I buy them the cardboard scratch pads that lay on the floor and are covered in catnip. The last couple of times I have put out a new pad with fresh catnip - one of them is urinating on it (not sure which one - I haven't caught them in the act!) My only guess is that he is marking his territory...any thoughts or experience with this? And if they are marking (and are neutered) what can I do to prevent it or decrease it?
Thank you!

i think you are right about that. I had a cat do the same thing for awhile. When cats scratch , they are marking their terrritory so the one peeing probably is just trying to inforce that is it HIS scratch pad

if it continues to anything else that he starts peeing on, then I would have him to the vet in case it is a urinary tract infection.