The Mystery of the Vanishing Coolant...

A couple of days ago, I turned on the heat in my Roadster for the first time ever (I've just been using the seat heater so far, but the wife was in the passenger seat, so…).

Anyway, this morning, I noticed that the coolant level was suddenly lower than usual for the first time ever. I waited until the coolant pump stopped, and where the level has always been just below the high fill line, now there was just a tiny amount at the bottom of the reservoir.

Coolant is still pumping, and the battery temp has remained cool. I can't find any sign of leaked coolant in either of the places I have been parking.

Is this a coincidence, or is there an additional volume of coolant that is used for heat, or something else?

I've topped up the coolant and will keep an eye on it, but thought I'd ask if anyone here has seen this before.

Mystery solved!

I wasn't losing coolant, after all. It turns out that the reservoir is much smaller than the volume difference of the coolant between high-normal operating temperature and really cold temperature, and you shouldn't check the coolant until the battery is at least off of the first line in the VDS temperature display.

The weather here in Maryland has been all over the place in the past few weeks. Near freezing when I first noticed the problem, then above 20C for several days, and now back down to near freezing. When the Roadster sits outside for several hours at near freezing temperatures (doesn't need to go sub-zero), the battery coolant level will appear to be frighteningly low.

Because I topped it up when I first saw the problem, mine is now over-filled (but I can only see that after a fairly long drive).