I know there are many who have experienced unusual cooling issues with their XF's and I have too, but I've never seen this one.

Reservoir is low, add coolant to top up, drive around & everything is fine, run errands. Stop in several parking lots, make multiple trips & stops. Never overheats, never complains.
Arrive home, park in garage, XF leaks a gallon of coolant on the floor, but always about a cup full at the bottom of the reservoir. I checked for codes, p0128, assume thermostat is stuck open. I'll order a new one & replace it asap.
But the real question is, why is coolant dumping all over my garage floor? I can't find any nicks, cuts, or anything that points to a source of the leak. I already replaced the dreaded line at the firewall into the throttle body.
The one at the back of the engine between both cylinder banks is fine too.

Could it be heat soak causing a hose to collapse, back off it's mount and let it drain out a bit? If so, that would be really irritating.

Starting to think it is possessed, might have to fetch a sage. (I'm kidding of course, it's been a very dependable vehicle.)

 

Only way to find a leak is to run a pressure test when it's cold. The SC 4.2L has around 23 cooling hoses and at this late date they all should be changed if you plan to keep the car. Are you aware of the 4.2L SC hose under the SC? Has this been changed?
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Thanks for the suggestions, I forgot to mention that I've run 3 seperate pressure tests while cold with 2 mechanics confirming it. All the hoses were changed about 4 years ago, including that one. Such a strange situation...

 

OK great news on all those hoses!
That is an odd one then. Not sure how you would find it when everything is hot. It's such a big leak it should leave behind some kind of evidence?

By chance has the degas bottle cap been replaced? What pressure version are you running?
One suggestion? What happens if you leave the cap cracked a bit so no pressure builds up?
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Have you actual measured the engine coolant temperature( ECT) while you are driving around? Most diagnostic tools allow you to monitor the ECT while you drive around and you should see a temperature in the 186F to 200F range. You could also see what happens to the coolant temperature when you arrive home and shut the engine off.
According to the Service Manual another possibility for P012800 is that the cooling fan could be running flat out and overcooling the engine.
If your diagnostic tool does not do PIDS you can get a device which allows continuous monitoring of PIDS. I have attached a picture of the device on my XF 5.0 NA which just plugs into the diagnostic socket.

 

I replaced the reservoir as the sensor had failed, and it came with a nice brand new shiny cap that is also holding pressure, I think its 16psi, I haven't tried leaving it cracked a bit, might do that. I have to replace the thermostat anyway, so I'll try that next if it doesn't work.

 

I have a scanner that reports live data, I'll check it out after I swap out the t-stat. Not sure why I didn't think to do that lol. Thanks!

 

I think that's pretty definitive. Ignore the purple, that's just from my screen when I took the photo. I replaced it with an OEM one and it's already back in the car, just have to button her back up and throw the scanner on while I take it for a drive. I'll post the outcome. Let's all hope this ends the mystery of the huge thirsty kitty marking her spot.

 

Update,

As much as I love it, this car makes no sense. The scan tool reports temps are good, hovers between 195 & 205 or so, fan kicks on when it's supposed to. It seems like the pink garage flood has ended (Knocking on wood). But now I'm even more confused. I took it out for a fairly uneventful drive, and it was quite hot outside today. Got it home, everything is fine. Sat for the next 10 hours, no leaks, coolant is still hot & IN the reservoir, not boiling mind you, but it's around 120f. I mean I'm glad it's IN the reservoir and not in a puddle on the floor, don't get me wrong. The issue for me now is where was it leaking from? I still can't find any evidence, anywhere, does it have a secret place where it relieves itself? I think I may consider adding dye to the coolant or perhaps try a UV light to see if I can find anything. So strange. Thanks fellas!