"compression test(?) on radiator"
Yep, they pressure tested the cooling system.
Sometimes there can be a "slow leak" when water leaks into the combustion chamber, hence there is no water apperas in the oil (evaperated), Copper gasket take more compression, also thicker (generally), do a search for more info. As I said before the gasket will get NO better, only worse, you risk damage if you it gets worse. Compression test would come up ok, as would a leak down test.