Funily enough my 1.8 190 is doing similar things.
I flushed out & refilled the whole cooling system about 2 months ago.
Driving home the other day the low coolant lamp came on & I had to top up with about 3/4 of a litre.
I put it down to air in the system when I refilled it, but my cap has seen better days. I now realise that there is a spring in the cap which "lets of steam" at a certain pressure, even if the rubber seal is still doing its job.

Hopefuly if this a common problem it will be solved with a new cap.