Water Collecting Under Wall Below Navien Tankless Hot Water Heater

We've noticed water collecting under the baseboard below the Navien Hot Water heater. The problem is intermittent; it isn't constant, and it's a small amount of water. We've noticed the water collecting during the day, when water is being used in the house, and when the heating system is more likely to be running. As an example, there was water on the floor along the baseboard yesterday. The water had dried up overnight.

Although the water is below the tankless hot water heater, it isn't coming directly from piping on the hot water heater itself. We've checked the hot and cold water piping, the drain, and cycled the pressure relief valve (in case water is splashing), and found no leaks. The water is definitely collecting inside the wall.

Water is collecting along the baseboard below the Navien
tankless hot water heater. Notice the vent on the wall.
Cold air pours through this vent during the winter.

The water runs all along the baseboard.

Drains (black), main water (blue), and plumbing
for the tankless hot water heater

Drain pipes

Intake and exhaust for tankless hot water heater. Line above
white pipes is HVAC power and condensate drain.

Exhaust goes into garage

and up garage wall

up through roof

Installation of vent ducting.

Installation of Navien tankless hot water heater.

Navien tankless hot water heater as it stands
today. Line above system is condensate drain
from furnace and power to HVAC units outside.

Condensation line exiting storage room to outside under deck.

Condensation line drain by AC unit.

Given the situation, the best guess as to the nature of the problem is condensation forming on the cold water pipe inlet coming to the Navien unit from inside the wall. There's also possibility of condensation forming on the exhaust pipe from the unit.

Pipe insulation can prevent condensation forming, as the surface temperature of the insulation will vary from the surface temperature of the pipe. Condensation will not occur, provided that (a) the insulation surface is above the dew point temperature of the air; and (b) the insulation incorporates some form of water-vapor barrier or retarder that prevents water vapor from passing through the insulation to form on the pipe surface.

Of course, another possibility is a slow leak in one of the water lines. The fact that the problem just showed up in the winter cold weather seems to mitigate against this, but doesn't rule it out.