sure thing
i'm pretty sure it's probably the filler/dipstick cap.
i just mentioned the engine because i think the hyd pump is mounted on the engine on these machines and if the pump has a bad seal and the engine has a condensation problem and by some strange chance the system has suction on the seal it may be drawing the contaminated oil in through that seal. Yep that sounds crazy but it's not impossible. Something that weird though you would definitely be seeing issues with the engine oil and possibly the coolant too.
you didn't answer my question about if it sat outside though?