I never corrected Mulder for marking in appropriate places. The once or twice he marked indoors (once at the vet’s, once at training class - in both cases I suspect he was not the first) I did correct him big time for it though, same as I would any dog that age who was peeing indoors. As he got older I got good at reading the way he was sniffing things and I could tell if he was thinking about marking something, in which case if I didn’t want him to (like too close to the entrance of a building, etc) I told him leave it and that was enough. In the beginning though he did catch me by surprise a couple times!