S80 low coolant but seemingly no leak?
 

I have a 2000 S80 T6, runs like a dream but about 4 months ago my low coolant light came on. The car never ran hot or anything but as soon as it was put into drive the light would come on and stay on for about 5 minutes then go out. After about 3 days of this I bought some Prestone 50/50 and topped it off which barely took anything because it was BARELY below the max fill line. As soon as I did this the light went out. Its now been 4 months and the light came on and stayed on for about 10 minutes each time I started the car. I check and this time the coolant was about a quarter of an inch below the min fill line. However I cannot see any leaks and the pavement is never wet under where I park everyday for work. Its now been about a week since I filled the coolant so I'm going to check tonight and see if it's still full, but again it doesnt seem to be leaking. I checked the oil and it is not milky so I don't believe it's the headgasket and also the heater is working perfectly so it isn't the heater core. Anyone have any suggestions?

The coolant may be dripping into the skidplate or similar underbody shield, preventing it from spilling on the pavement until the car starts moving and tilting

Check the driver side carpet if wet. Lift carpet up and check the heater core u may have a small leak. If not there ur best bet is to pressure test.

As well as the heater core, it most likely is coming from the lower right hand side of the radiator, you can see where it has dried up without taking off the skid plate.

I'm having the same issue wth my 99 S80. The coolant seems to be disappearing. When I stop the car and turn it off, the coolant reappears in the evaporation bottle, then continues to rise until it spews out from under the cap. I plan to replace the cap this week, but I still don't know where I'm losing coolant. Car runs fine, oil looks good, ...it's a mystery so far. Still looking for suggestions.

any updates to this ? i have a similar problem

Same issue here. Car is taking about 1/2 quart of coolant every other week. Pressure tested twice, passed both times. Tightened all the clamps around the overflow tubes as there was some residue there from prior moisture. I noticed a hairline crack in the green filler cap and just replaced that with OEM. I'll see if that does the trick - but even so, there has never been any moisture underneath the car or on the floorboards.

wow alot of the same problems
 

well mine is prob a lil different I had replaced the reservoir & sensor & I am getting a low coolant light but my reservoir is always at the same level I have removed the sensor & placed it again & it will only take a matter of maybe a day or so & it comes back on no leaks & I'm just not sure what it is. not to mention since i got my brakes done my ABS light comes on after ridding on cobble stone rodes or bumps which i guess I have to determine which wheel sensor is bad since my ABS module was changed 2 years ago on the 4th of July. I think mine may be more perplexing since i have no leaks or fluid loss

Here my comments,from a problem that I had with my wife's 01' S-80. She was having overheating problems without losing any coolant. I replaced the thermostat first, then the radiator with still no resolution. The car would run hot from time to time. The water pump had been replaced by a high end shop 30K miles before this problem so I did not suspect the water pump. I decided to pull the pump and redo the timing belt which had been done during the normal maintenance to find out that the impeller which was plastic was spinning freely on the water pump shaft which was metal. Boy was I pissed. I got it all back together with a new metal impeller and metal shat pump and Filled it with coolant. It took about 3 weeks of driving before all of the air was completely gone out of the system. I kept adding fluid to the system a little at each time till she was full. it actually took 3-4 weeks of driving for it to burp all of the air out of the system. Don't know if that is what you are seeing but that is my story. Good luck with your lost coolant. BTW I have a good used radiator from our S-80 that is in good clean condition that I'll sell cheap if anybody is interested.