Old boy suddenly weeing in house

[TonaldDrump]

I'm wondering if our nearly 13 year old boxer-lab cross is showing signs of dementia.
He's been house trained for years and only sometimes made a mess when stressed during house moves and new baby.
Lately on a few occasions he's just weed in front of us. Didn't ask to go out and been in and out all day. Just stands there and wees. It's like he's forgotten he's house trained!
He's still his usual happy chappy self and was fully checked out as totally healthy a couple of months ago when he had an op.
Could this be the start of dementia? What else could explain it?

Maybe take him to the vets again specifically to check for bladder infection etc

I was reading about that but since I've got up close and personal with his wee, it's perfectly crystal clear

Definitely ask for a vet appointment. Possibles include a UTI, kidney disease and even cancer. Problems with hips/arthritis might also affect continence.