Oh the old Frost proof hose bib caper where where there are no leaks UNTIL someone opens the frost proof faucet and had a split in it as the home owner neglected to disconnect the hose (garden) and allow the bib to drain as designed HUH?

:-))))))

When we first moved in the old lady we got the house from told us not use the faucet on front house that it leaked in the closet. She may have left the hose out for the winter we don't know.

I would open the wall and then expose the piping then have someone open the bib and have a light hammer ready to tap on the pipe once you locate the actual leak.

Then once it is found then a course of repair can be discussed

GOOD LUCK