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 Miniature Schnauzer, watch their appetite and their toilet habits. If a dog this keen on food suddenly will not eat, vomits, or hunches over a sore belly, contact your vet the same day. Straining, peeing often or blood in their urine can be stones rather than an accident indoors, and for that, contact your vet the same day too. An older Schnauzer who drinks and pees more, or loses weight while eating well, may have diabetes, so contact your vet the same day. 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.
They'll keep reporting the corridor. If they go quiet and turn down their dinner, contact your vet the same day.