Reservoir for the radiator keeps emptying out below the full level. No leaks on 2006 Ford Explorer

are evident. I did lose heat once. What could this possibly be?

3 answers 4 comments
Have it pressure tested, and the cap tested.
Thank you for your reply. Our mechanic said he did notice radiator fluid on the radiator and I think he said the engine too but he couldn't find a leak. He couldn't get a reading when he checked the pressure. I hope this makes sense.
No pressure drop? Might check with engine running to make sure as well, engine at operating temp. Suggested cap, might not hold rated pressure, coolant on radiator is a good sign, just want to be sure engine isn't building more pressure then rated capacity of cap pressure rating. Thanks for checking back.
Steaming from exhaust or any misfire? Misfire on cold start after sitting overnight?
No steaming from the exhaust or misfire. Sometimes the engine will rev on cold mornings but this isn't consistent at all. Someone suggested a leak in the radiator hose but we had our mechanic look at it and he couldn't find a leak.
pressure test system to determine source of leak if any then repl rad cap
Thanks for your reply. We haven't tried a new radiator cap but maybe we should. Pressure check didn't show anything when our mechanic tried this.