Unknown device or appliance help

This appliance was observed on an exterior wall embedded in the stucco in San Clemente Ca. I’m not sure that it is a plumbing device but I don’t recognize it. Does anybody out there know what it is? The other side of wall at the interior is drywall.

Not sure…do you have a picture with the cover removed?

Tankless water heater

tankless gas wh
warmer climates mount in exterior wall for direct vent

Interesting, I thought it looked like the exterior door of my rv water heater.

Did you open the door? Could be a small water heater for a single bath…but without more info kinda hard to tell.

It seems like tank-less water heater…