Dog peeing in bathroom

We adopted Spook, a cocker spaniel June 1. Our other dog had died in May. Spook is supposed to be 3 according to the shelter where we got him although our vet says he doubts it, and groomer said maybe 2. A month or so ago, he started peeing in our bathroom. In one case he had just been out. I am at home most of the time and he can go out anytime. However he has to stay in his pen as he runs away.
Any ideas why or how to end it?

Mary

Have the dog checked for a UTI if this is a new behaviour. Shut the bathroom door to keep the dog out of there if that is the only area he is sneak peeing in.

If it's sudden, a vet check is in order, a medical problem could be causing him difficulty holding it.

Make sure you've cleaned the bathroom floor and any walls he's marked on really thoroughly, with vinegar or a good pet stain remover, so he doesn't feel the need to mark, and don't let him in the bathroom unsupervised.

Thanks for the ideas. Have tried most, but not vinegar.
Mary

I want to thank everyone for their input. Took my dog to the vet and he does have a UTI. Will be on antibotics for 10 days and then we will see if the issue is resolved.
Mary

Glad you found out what it was. Was going to post it sounds like when I first got Yoshi and she had a UTI.

Just make sure to use all the antibiotics. Once it was cleared up she didn't have any accidents.