230E losing water slowly

[simmo300e]

Our 230E is using water at the rate of around 500ml every week or so. There are no apparent leaks and the hoses look Ok. The coolant tank cap has been replaced. The water pump doesn't appear to be leaking. Also the car runs hot sometimes, up to 100 on the meter when in traffic but drops to 85 when running on an open road.

Any thoughts/suggestions on where it might be losing water?

Also, as we are in the tropics, the cars are running with plain water in the system, which is not something that's been commented on or changed when they go to the dealer for a checkup/oil change. Is this Ok or should I be using a coolant mixture anyway?

Your help, as always, would be greatly appreciated.