Adult dog weeing inside suddenly

So Sarge is about to turn 5 in a few weeks. Male staffy, and he still has his 'bits and bobs' hanging there on the loose.

We have had him since he was 8 weeks old and up until recently (maybe the past 7-8 months), we have had no issues with toileting (apart from a few accidents). Up until recently, he's been good and has always relieved himself outside somewhere.

The last few months, we have been finding he's been peeing up the back of the lounge, the side of a storage cupboard, and on a particular wall up the hallway - it's almost always in these same few areas every time, with the odd areas on occasion.

-He has access in and out of the house through his dog door (recently installed within the past 6 months). He has his freedom throughout the whole 3 level house (if we are home) because we have a few people living here, and most of the time we're all just in our own bedrooms, so he likes to kinda bed hop between each of our rooms. If we're all out in the living room or kitchen around dinner time, or in the morning, he will just hang out with us there.
-He has his own little area outside on the balcony with his kennel out there, and is kept outside during the day if no one is home, and as soon as someone gets home we open his doggy door up so he can come in and go out as he pleases.

-Most of the time, these incidents occur at some point during the night, and we find it when we are up in the morning, however, there's been a fair few times during the day if we're home when we are just in a different area of the house. -He will never do it in front of anyone, only if we are not around at the time.

-I know that he knows to use the dog door to get outside to toilet as I have watched him use it multiple times. He will suddenly get up - go outside through his door - toilet - and then come back in once hes done.
-Before we got the dog door he would hang around the sliding door to get outside if he needed to toilet, and if it was locked and he couldnt open it himself, he would whinge or somehow get our attention to let him out. He still does this sometimes if he's downstairs in the very bottom-level bedroom and needs to go.

-He gets adequate exercise and we haven't changed his routine for years and on top of that we also take him out the front to the grass area first thing every morning and before bed every night, as well as multiple times throughout the day if we can, for a run with the ball and so he can go to the toilet if he needs to.

-Nothing has changed in the household, no new people, no changed routine, and no new furniture (and even when we have had new furniture he doesn't even look twice anyway), and we haven't changed what we feed him.
-We don't own any other dogs or pets, and haven't had any other dogs in the house - even visiting.
He has been to the vet since this started occurring for his usual checkup, and no problems in that area, so it's not a UTI or anything.

-I also get the whole 'marking territory' thing, however, I just can't figure out why after 4 years of being perfectly fine he this would be the reason?
- When he does wee in the house, he basically dobs himself in before we even find it/know hes done anything. He will either go into someones room and not follow when we walk into the kitchen/lounge area (where it usually happens), or he will just look guilty as hell if someone is anywhere near the area hes done it. Doesnt matter if he only recently did it or even did it the night before, he remembers and will act all guilty until its cleaned up.

- Iv also tried the deter sprays but they dont seem to bother him either.

- And don't know if this is relevant, but I'll just add, we do also find he leaves a number 2 inside on the odd occasion, but usually it's when it's raining i think, and he just wouldn't want his paws getting wet (little princess when it comes to getting wet).

Any advice, information or thoughts id appreciate! thanks in advance

Well, my first thought is a bit hin heat nearby?

Other that I would ask the vet to check for prostate problems.