It'd have to but how. I know there is oil (probably splash engine oil) on the pump's drive gear (piston pump on left side). Would have to have some kind of pressure and really bad seals to get into hyd that way. Hyd lines to the hyd section don't come anyways near the crankcase. Bolt up to the tranny. The rear main seal is out as there are too many things to stop that from being it.
How's the hyd oil level? Does it appear to be higher than before? Does it have remotes? If so, dirty oil could be coming from dirty implements.
Man you have a brain teaser here. Will think about it this evening and see if I can come up with something.
PS Guy doesn't have neighborhood pranksters does he?