Dog peeing inside

My rescue dog (6 years old) has never been officially potty trained, however he is a pretty clean dog and does not just pee on a bed.

We have a special section in the house for him it is pretty kennel like and yesterday night he peed on my bed in the room. It was a full on pee and not just marking his territory.Usually he just pees in the outside section.

Should I be worried or is it fine if its a one time thing?