
Hi Jim! Did the people who lived in your house previously have dogs? If so, and this marking started only after you moved in, then it's possible that your boys can still detect a lingering smell of those dogs and need to cover up the scent with their own to feel more secure in their new territory.
What do you use to clean up after them? Anything containing bleach will only encourage the behaviour because it contains ammonia and, to a dog, smells similar to urine so again needs to be covered by their own scent. You mention odour suppressants, but what sort? The best thing to clean with is a solution of biological washing liquid followed (for hard surfaces) with a wipe-down with vinegar, white spirit or surgical spirit to remove any lingering smell of urine.
Hope this helps.