evaporating rear differental fluid??

My 2006 4 x 4 double cab with 55000 miles went for dealer service for a knocking noise. Apparently, the rear differental was way low in fluid. I've never serviced the differental or had any leaks. The dealer is telling me it evaporated...never heard of gear oil evaporating...it's a sealed unit. I've searched the web and found both answers that it can't and can evaporate. Any toyota mechanics know the true answer?
Thank- new to forum guy - Joe.

 

It is not a sealed unit. There is a vent in the top.

 

What he said. There would have to be serious issues to create that much heat, and the remaining oil would be very dark and smell badly burnt. Sounds like it was never filled correctly from the get-go, but, the level should have been checked as few times during that time as well.

 

I agree with the others about this stuff not evaporating.

Dare I ask, how well do the brakes work?

PK

 

The truck was purchased new and no maintenance was preformed on the rear by Toyota service or myself. The truck was scheduled for a differential check at 30,000/45,000 miles which I regrettably shouldn’t have disregarded. Seems I pay more attention to my classic cars then I do our daily drivers...I obviously need to work on that. But again- no visible wet spots on the rear or pavement...my bad.

The service manager has agreed with my argument that gear oil doesn’t evaporate and was likely never filled correctly from the factory ( as landphil stated). The mechanic and service manager also agreed the vent wasn’t a factor in fluid “evaporation”. Since my miles are typical driving and not “special operation” (towing or off road) they no charged the flush and fill. The mechanic felt no permanent damage was made on the rear...since it was caught early... but only time will tell. Good news- The knock is gone... However, to some of your points...friction, heat and corrosion are certainly factors in gear oil break-down.

Thank you for the quick and educated replies. I appreciate those who eloquently responded without the need for sarcasm.

Thanks again,
Joe

 

And Welcome to TW