low on coolant but not overheating

I drove my car around today and was checking fluids after. It was low on coolant in the resivor so i checked the radiator cap and it was not hot so i opened it and the radiator was low on coolant, so i filled it and watched and flow was good through radiator when it was running. The hoses the water pump and thermostat are all very new (about 20,000 miles or so since water pump was changed with timing belt) about 10000 for hoses and thermostat. It was not over heating anytime i was driving it and has not overheated in recent memory. It has no fluid leaks on the ground an no deposits that suggest fluid leakage of any kind in engine compartment. It does not smoke and i keep up with all maintenance and i am anal about staying on top of any and all issues. Any ideas to try?

Did you burp it last time you refilled it? It might not be that you lost the coolant, but that you just never had it in the first place.

yes it was burped last time it was filled, oddly a few months ago i checked and it was full and has been everytime i checked until recently. wouldnt air in the system also cause it to overheat?

I'm diagnosing similar sudden coolant loss on my 2003 P5. Rate of loss seems to vary. I've done radiator pressure check (kit bought at Harbor Freight for $69, anyone want to buy mine?), compression check, all OK. No evidence of leak in heater core. I'm changing water pump and timing belt (first time at 102K miles).

mine seems to lose it very slowly, i only had to add about a quart total to fill it up. i think i have it down to a radiator cap, because obviously if the water pump was bad it would either use a ton of coolant and/or leak and overheat mine is not doing any of that the hoses are new and not bloated they are tight and there is no tale tale sign of coolant leak(white or rust colored deposits) anywhere and no drips, i checked to make sure the system was properly burped just now and it was, it had good flow through the radiator and didnt release any air bubbles. but i did take the cap off while it was warmed up and it did not spray coolant or feel like it was pressurized as i was turning it, the cap is fairly new also however i think it has become faulty and my culprit. i will try that (i work at autozone so ill just buy one while i am at work later) and report back with findings when i get off.

I'm not an expert, but I thought only way a bad cap would lose coolant is by boiling over (without pressure boiling point is lower), but you said it didn't overheat. I'm not sure a bad water pump necessarily does leak a lot, but I agree that it might. I really can't see any leak evidence around it.

By the way, another neat trick I learned is to pump up the radiator (17 psi), unplug the coils and crank the engine. If pressure fluctuates, it's the head gasket.

i know my head gasket is fine, it has not overheated ever except for about 60 seconds when my thermostat went out as stated in my original post all cooling system parts and fluid(other than radiator) have been replaced recently (within 20,000ish miles) it also does not lose oil or smoke, and the oil is not looking like chocolate milkshake. there are also no deposits or leaks coming from the engine anywhere. the new cap seems to be working, its holding coolant and the cap is heating up when the car is at operating temp like it should will post here again if it starts acting up again. i may replace the coolant temp sensor next if it loses coolant again. but i am almost sure the cap was the issue as compared to the new one the old cap had a stiffer feel and may have been getting stuck open and seeping just small amounts of coolant out that were gone before they dried.