What are you using to clean the area with? Anything with a harsh chemical smell, ammonia or bleach may encourage the cat to cover that smell up with their own. Ammonia actually smells very much like the base smell in cat urine, so if the previous tenants had a cat who peed in the basement & they didn't clean it with an enzymatic cleaner, your cat may be covering it up or thinking that's an acceptable place to urinate.
In order to remove the odour the cat can still smell even if you can't, you need an enzymatic cleaner like Nature's Miracle or the homemade version.
Homemade enzymatic cleaner: 1 cup water, 1/4 cup white vinegar, 1/4 cup isopropyl (rubbing) alcohol, mix together in a clean/new spray bottle. Spray the area affected, blot with paper towel/clean rag, spray again & let dry.
You can also use this to clean the litterboxes, garbage cans etc.
Perhaps you can add 1 more litterbox & see if that helps, some cats prefer to pee in one & poop in another. You could leave the Pine Pellets in the current box & use the above mentioned Cat Attract litter in the new box & see if those 2 additions makes a difference.