Swapped out the cap on Tuesday for $6.50.
Checked the coolant level today and it is now consistent!
Interesting side note - on Monday night, just 3 minutes into a 30km drive, the coolant light came on.
Stopped the car, turned engine off and opened cap to check the coolant level but it was good (coz it was just checked and topped off in the evening).
Started car and coolant light went off and never came on again.
I put this down to dirt/crap on the low coolant sensor and will take a look at it when I have the time.
I believe it needs a good cleaning.
Also, the base of the cap, on the tank, may be leaking. Not the cap itself. In this case you need a new tank @ $40 (or used one)
Kind of a strange coincidence but my kid brother's 190E and needs a new cap/tank coz he is losing coolant from both the overflow hose as well as the base of the cap.
This was an easy one.