If they replaced only the pressure switch or compressor, they shouldn't have (what'* with this cough stuff) touched any of the rad stuff. Checking that hose is a good idea, that'* more coolant than a bad burp job though.
Please read the link I posted, being a Series 2 it'* info you should know and keep in mind.
To my knowledge no one has ever found visible water in their oil when the upper intake has failed. However..we have seen bearing damage therefore water must have gotten into the system while not being obviously visible.
00 Regal GS: All kinds of cool stuff.