I had a similar situation. Coolant level was at max when cold, but the low coolant light would come on after about an hour of driving. This may be how long it took for coolant to circulate through the auxiliary radiators or long enough for it to leak enough to trip the warning light. I think there is an electric water pump for the aux rads, so they may switch on after some condition is met (like coolant temp above x). My driver side aux radiator had a leak. Easy to check if you pull the wheel arch liner. The common leak starts at the top. Seems to be an issue with early p300s, not sure what years are affected. Saw a post that the 2024 p300s have deleted the driver side aux radiator. I guess thats one way to fix it.

 

About a month ago, this started happening with my '22 P400 at around 52k miles. Happens 1-2 times a week when the engine is hot and clears after the truck sits overnight. When it happens, I always check and the level is always the same - near the max level, maybe a bit over. I figure it's a bad sensor.

 

It's exceedingly worrisome to me that a service advisor would send you off saying it's normal to to need to top off the coolant.

I say this because the system is a closed lopped. Meaning there is no reason for consption of coolant.

I admit I could be wrong here as I'm no mechanic. However, for there to be missing coolant it must be leaking from a hose or radiator directly. It just doesn't get 'consumed'.

 

Its a closed system it is absolutely not normal to have to top it off. I just had the low coolant light come on this weekend. Took it to the dealer and it is a leaking hose going to the turbo charger. First time in four years it needed coolant (well except when I drained the aux radiators when I relocated them for larger tires