DA40-180 G1000 Alternator warning light mystery

Little help from the list:

I've had two shops look at my DA40 and no luck. Two weeks ago I flew
pretty heavy IFR long x-c (Leesburg, VA to St Loius, MO) the plane sat
on the ramp in the rain overnight one night. Next morning the
"Alternator Failed- the battery is the only power source" red warning
came on.

I immediately went into the shop where I was and had them look at it
and put a Fluke on the connection just in front of the firewall. This
is after looking at the alternator, checking the tension of the belt,
making sure everything was dry,etc. I continually watch the system info
to make sure the vlots are 28 +/- when the engine is at cruise, so I
know the alternator is working. The plane seems to be running fine, but
it's not a comforting warning light or thing to have dinging constantly
on approach.

The warning light now predictably comes on during most run-ups, and
stays on for climb out and back on at decent. This makes me think a
sensor is overheating or something.

Anyone have any ideas or similar issue?

Thanks.

Sweendawg