Bongo Fury

Had my bongo 2.0 in for a few bits and they inspected thermostat. Didn’t get chance to replace it as part didn’t arrive but thermostat housing went back on, with a bit of sealant for good measure. No Leaks there that I can see. However coolant has been very slowly (1/2mm) reducing when I use it. Could it be stabilising since the housing was removed due to air getting in there at the time?

In the past weeping from the water pump (at least on the diesel) has also been a cause as it only happens when the engine is running, not sure if this could be a similar problem on the petrol 2l ?