[kaiser]

I don't think that would be the first option. 150ml per week is not a lot. I don't think any compression test would pick that up.
Except perhaps a compression of the cooling system!

I would check the outer spark plugs for signs of water, by-pass the oil cooler, and keep a weary eye on the V for leaks from the plastic brigade.

Then see if that helps.

Don't forget you can leak water from the water pump as well! That will show near the crankshaft pulley. Look for colour change, not necessarily wetness!