Clean it and dry it off and check it again when under pressure as things have a way of weeping when at operating temp. Could even be weeping down from further up and running down
eg; A woman at work was baffled why her coolant kept dropping, V6 Commodore,she would fill it back up each arvo when the car was cool prior to going home, checked it in the morning, when cold all was good, but it was always low in the afternoon when she was about to head home.I had a look for her, started the car filled it back up, opened the bleed screw on top of thermostat got all the air out, I then asked her to turn the heater all the way on after it started to build up pressure and there was her problem the bloody heater tap had a small split and when the engine got up to temp the split would open up under pressure and it would squirt out, no tell tale signs of coolant as it would spray out on the exhaust manifold and simply evaporate as steam, and she didn't use the heater on the way home only coming into work in the morning.
Went to a couple of mechanics who diagnosed leaking inlet manifold gaskets, still going good to this day after a new heater tap was put on
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