I've been through this myself, so I've done a lot of research on the subject. Cat pee stench is horrid. I'm sorry you have to deal with it.
A cat who has only recently started eliminating outside of its litter box is likely suffering from a bladder infection or anxiety. I cannot emphasize this enough: It's very important to take your kitty to the vet asap and make sure he/she doesn't have a bladder infection - they are often fatal, especially in male cats.
Anxiety-based peeing is not fatal, unless you count your desire to kill the cat every time it pees.

Cats are creatures of habit, and if there's been a disruption in his/her routine or environment, he/she may be peeing on your things out of stress. The disruption could be a new litter box/litter, a different type of food, a new smell, an old smell left by another animal, a rearranged living room, the arrival of another cat within smelling/visual/auditory distance, a change in your own schedule that puts you home more or less etc etc. It can be maddening to isolate the source of your cat's anxiety because it can come from just about anywhere.
Also, have you noticed a change in his/her clawing habits? Is he/she clawing a lot more furniture, carpet, YOU, etc? Compulsive clawing often accompanies inappropriate elimination as a sign of an anxiety disorder.
Cats also like small spaces. They have an urge to stake their claim on their 'territory' and then patrol it. A kitty who is peeing outside the box may have too much 'territory' to cover and is once again, stressed and scrambling to keep their overwhelmingly big territory. I know you are adverse to keeping your cat in the bathroom while you're away, but it might be a stress relieving on the cat to shrink his environment a bit.
And finally, some cats develop a generalized anxiety disorder not unlike that found in humans. That was ultimately the case with my kitty, and medication was the only thing that helped. Medicating the cat with anti-anxiety meds (like the colompramine I give my cat every other day) will in most cases greatly reduce or completely eliminate his/her urge to pee outside the box.
Once a cat starts peeing, though, they will remain attracted to that scent and it will be difficult to get them to stop, even with every measure taken. I moved from an apartment with lots of animal smells from previous tenants into an apartment with no animal smell, and it's been further calming for my cat. I've had no peeing incidents since I moved.