"Low Coolant" idiot light

Hi all-

Lately the "Low Coolant" light has been popping on right after I start my car in the morning. It usually only happens on cold mornings (30-35 degrees, cold for Northern CA). It never happens in the afternoon, after work when the car is cold but ambient temp has gone up. I checked the puke tank and filled it to the "full" line, and the car doesn't seem to be running hotter.

Yet this problem persists. What do you think? Sending unit or sensor of some type maybe?

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Greg
91 LX Convertible