Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Standard Schnauzer, watch their back legs and their energy. A young dog who sits down on walks, hops on their back legs or is slow to get up may have sore hips. A Schnauzer who is heavier, slower and less interested than they used to be may have an underactive thyroid rather than a lazy streak. Gums that bleed, or a cut that will not stop bleeding, mean a call to your vet. Later in life kidney disease joins the list, and drinking far more than usual is worth a call too. Bloat (GDV, a twisted stomach) can happen in any dog, though it is most common in large, deep-chested breeds. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now.
He'll keep flooding the kitchen. If he starts drinking twice as much, call your vet.