Consider this possible explanation:

You are in cold weather, right? However the outdoor air still has a little water vapor in it, maybe even close to 100% relative humidity. Air and engine cool off over night, right on down to whatever the low temp is that night.

But the metal mass engine cools quicker. The air still holds X amount of water vapor. As it cools, it cannot hold as much water vapor, and air near the intake manifold cools quicker. So that air must give up some of its water vapor, and it does - in the form of condensation.

The intake manifold is flat and silver, and about the highest part of the engine, right? Other engine parts may have condensation but not so obviousmly.

That's my explanation. Don't make any bets yet, someone else may come along with a better one.

Good Luck,

Bob

:>)