most likely chronic anxiety... spite is not a dog's repertoire of feelings. Maybe he does not like being left where he is... might see your vet about maybe trying him on some anti anxiety meds, or your trainer about suggestions of how to train him out of this behavior. You can also purchase wee-wee wraps for male dogs (to keep them from marking everything).. .they wrap around the body and when they pee, they just pee into the wrap... have to be changed frequently, though, and can cause a rash if not cared for properly.

How often does this dog get leash walked?
When you let your dog outside to do its business, do you just let it outside to wander about on its own, like in a fenced backyard? Do you go outside with it? This behavior can be linked to a number of behavioral issues, which can be resolved with proper training which takes time and energy.
How long have you owned this dog? the whole 7 years or for a shorter period of time? What is the dogs day look like? How much time does thie dog stay outside? IS the lab male or female? I ask these questions because it might help me get a better idea of just what is going on. Medicating a dog that might be trained out of an unwanted behavior should be the last resort, especially since every medication has side effects and some can be expensive and not even take care of the underlining behavioral issue.

Have you ever had your dog's urine checked to see if he has a urinary tract infection? This would be my first course of action, and it has proven to be the root of the problem in 3 of my pets who began to urinate in the house at various times in the past. A simple course of antibiotics might be the solution. I would explore this avenue first.

How long have you had the pug? If he has been with you for a long time, and he has always peed indoors, then unfortunately he has become accustomed to believing that this is the correct place to relieve himself - sort of reverse potty training. In this case (after you have him checked out by the vet) you should go back to house training 101, using a bellyband and constant supervision, taking him outdoors whenever you see him sniffing or acting restless, and keeping him in a crate when you cannot be there to supervise. But be sure not to leave him in the crate for more than a few hours at a time, and being sure that he pees outdoors before you confine him.

I'm not a fan of the belly bands, and would recommend using a crate to retrain him instead but if he's never been in a crate before you'll need to crate train him first. At least with the crate he will be in a place he feels secure. It might be worth the expense to consult with an animal behaviorist or trainer.

We don't have the cage type crate, we use a large carrier and our lab has loved and considered it to be her space or "bedroom" since she was 6 weeks old.