Davej wrote

No water in one of the upstairs bathrooms this morning. Thinking of
cutting a hole in the wall to try to figure out where the coldest area is.

Makes more sense to use a thermal imaging camera. Not that expensive now.

In the basement I can see where the pipes go up. It
is an interior wall but quite close to the exterior wall.

Any suggestions?

Has that happened before ? Have you lived there long ?