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Does he seem to drink a lot?
Is your older schnauzer neutered?
Okay. This may be coincidental. I suggest taking the older guy in for a urinalysis. If the urine shows abnormalities like a low specific gravity, protein in the urine, blood, bacteria or glucose then you may actually have a physical issue going on. Schnauzers very commonly get stones in their bladder and infections are right up on the list as well. Another common problem seen in older Schnauzers is Cushing's disease which can also cause an increase in thirst and urination. If nothing is seen through the urine then I would have to recommend bloodwork. Before punishing him for having accidents I think given his age we need to first rule out any of the above physical issues.
If urine and bloodwork all check out then you are most likely looking at a behavioral issue. If this is the case then you will have to start by retraining him. In his case I would crate train him. It would be no different then if he were a puppy learning from square one. Take him out when he gets up, take him out after he eats, take him out after he plays and stay on top of him. Do not let him out of your sight for a minute. If you cannot be with him then you will need to put him back in his crate.
Do ask your vet about the possibility of bladder stones because this may or may not show up in the urine.