Mini One 2008: Losing water

[Scharxoxo]

I’ve been having problems with my car. I keep losing water while I’m driving it, it’s not leaking from the car and doesn’t move when the car is parked. Water loss is only from driving. Any help or suggestions would be greatly appreciated!
Thank you

[Alpinestars]

Expansion tank and lid are a weakness. Could it be that?

[Baldchap]

How does the oil look?

[GreenV8S]

It's fairly common for failing seals or pinhole leaks in heat exchangers to only leak when under pressure and/or hot, and stop leaking when the system has cooled down and lost pressure. If the leak is slow, and only occurs when hot, the lost coolant may evaporate rather than drip, so you won't see any evidence unless you happen to look right at it - which is nigh on impossible in many cases.

You might find that a hot pressure test will at least confirm that you have a leak, and if it also happens when cold then a cold pressure test may result in a drip which you can eventually trace.

Any unusual smells inside the car? If not, hopefully you can at least rule out a heater circuit leak. That still leaves a lot of suspects to check for under the bonnet.