TOPIC: Does anybody know what this is?

I was being facetious  . It does look like some sort of access panel, but don't know why it's louvered. Could it be a fresh air intake of sorts?

Michael S qoute from above:
"I have never seen one covered by a panel and I don't think that a tankless water heater would fit inside of a stud bay with 2x4 framing and not be visible on the other side of the wall. Are you sure this is a tankless w/h?"

Mike, I'm quite sure it's a tankless water heater. Here is some further verification w/ pics. The photos attached are from a New Home Pre Move In Inspection. Look familiar? As you can see not much depth is needed inside of panel as shown in the pic with panel front removed. It's not very common even around here (good climate) to have it at exterior but I have seen it.

Out of curiousity, why did you post your question in the plumbing section of forum, if you didn't know what it was in your pic?

Nice pic Joe!