Dog still wees in the house

[Sunnyqueen]

Hiya bit of background,
I have a 12 year old male dog, he is a rescue who I got at 10. When he came to me he was completely un house trained and had previously just been left to wee and poo all over the house his entire life. I pretty quickly got it down to I'd say 90% going outside but that 10% still really bugs me. I would say it's 1 wee a day usually. In the milder months so from April till October I leave the back door open all day, makes no difference he will still choose to go in the house. He has NEVER once done it in front of me he will purposely go off to do it in a different part of the house to do it. I have never caught him in the act once.
Walking seems to have no impact, there have been many times I have taken him on 2/3 hour long walks and he's done it within 10 minutes even though I have the back door open and as soon as we come home he will go out there... But then come in and wee in the house!

Any help please? I've had to change all the flooring in my house from nice carpets to laminate because I'm paranoid about the smell lingering.

Some dogs need to have the door closed, otherwise they get confused about where "outdoors" ends and "indoors" begins, so I would start with having the door closed 24/7 and training him to ask to go. You could use doggy doorbells, for example, and reward him for ringing them.

But the fact that he goes off and wees out of sight shows me that maybe, at some point, he was told off for toileting inside.

Are you cleaning up the accidents with a proper enzyme remover? That should help as it'll stop him from going in the same spot over and over.

However, all that being said - he's 12 and many older dogs start having accidents in the house when they've previously been perfectly toilet trained. I walk quite a few older dogs (12+) who have started toileting inside again. It is, to an extent, part of having an elderly animal.