My APBT is 13. He is arthritic but with his suppliments and diet he is in great shape. Friday night (when we are gone the dogs are crated, we have a dog walker come during the day), he was out (around the house) with me and my husband....he walked right up to us, looked at us (we were in the kitchen) lifted his leg and peed on my Husbands gun cabinet. I started to say something and before I could finish he ran upstairs into his crate.

I had him seen by his vet on Sat, he says the dog is in great shape.... Is this "old" dog behavior, or did we do some thing to "pi** him off" literally??????

Well, you know you're going to get yelled at for humanizing....LOL . But only he knows why he did it.
Is there any reason he might have wanted to mark the cabinet? Another dog in the house (even for a few mins)? I would think it was "old" dog behavior if he hadn't lifted his leg. Lifting is usually more related to marking than just peeing, in the house anyway. Is there anything else "new" that might spark him to reassert ownership?

Caleb's "Dad" (my ex bf, who Caleb was actually purchased for) stopped by a few mos. ago. Caleb hadn't seen him in a few years. I brought him out of his kennel, he marched through the house, barely glanced at my ex, then proceeded to pee on the couch, the chair my ex was sitting on, and a plant in the corner. As if to say "you can show up again if you want, but now, the b!+ch and her $h!+ belong to ME." I of course corrected him...not that he cared...he'd already left the house and went back outside. My ex observed that while he looked much older and more mature, he was still an @$$.

We will never know why they do the things they do. Never. Unless we can get them to use a keyboard or a pen. It's good that you had him vet checked, and glad to hear he's in such great shape at 13. I'll pm you; I got a bday card from "Giggly Wiggly."

Does the cold outside maybe make him ache so he decided to go inside?

Well there is a new happening in the house. My 20 yr old neighbor (a male) is now training with me. He comes over more often and Buddy loves him. As soon as the kid walks in he sits on his feet and gets pats on the head. This particular night the kid worked late and did not come over....hhhhmmmmmmmm

And yes it is very cold, but it seemed so deliberate, that is what took us by surprise. What also was frustrating was he had just been out in the back yard to do his doody...

Is it possible he's marking b/c of the new guy? Mine have been known to do that w/a new male presence.