Taking a Closer Look at the Leak

I removed the baseboard and about 3 inches of the drywall. This is what I found.

It is wettest underneath the vent. Insulation is wet there, whereas it is not on either side of the cutout. The photos below show what's behind that vent.

Tankless Hot Water System and Sewer System Plumbing

These pipes under the deck appeared Wednesday or Thursday last week. Eventually, a chase (i.e., box) will be built to hide these pipes.

 One is apparently a exhaust line for the tankless hot water, and the other would be the intake.

 The exhaust travels up the back of the garage,

 and vents on the east side of the roof.

 The pipes begin in the lower level storage room where the tankless system will be located.

There's another piece of large diameter pipe right under the Bedroom #3 west window

This is apparently a sewer clean out. This eyesore will be cut down closer to ground level and covered with a lidded box.