My mechanic recently rebuilt the rear differential with new bearings and reinstalled it in my 2005 Impreza Outback Sport. I didn't notice anything after a couple of short drives but ever since I dr=ove it about 100 miles round trip last weekend there has a been a pretty odor of differential fluid wafting from under the rear of the car, especially when it was garaged. Even several days later, the odor persists. 

Tonight, I jacked up the rear of the car and examined the differential carefully. There is not even the slightest fluid leak at any of the seals, nor was there any residue of differential fluid on the outside of the case.

My mechanic did use a heavier fluid than spec (75W- 140), but I can't imagine that would cause any issues. The differential case seems sealed, with no vents that I can see, so I'm not sure why I keep smelling differential fluid when I'm around the car. 

Also, there's no odor from the front of the car at all, nor when lifting the hood, so I doubt it's the front differential, but it really is stronger around the rear of the car. Also, I thoroughly checked in the back of the interior of the car to see if anything had spilled back in there, but nothing was found.

any chance you have a split cv boot leaking grease? when that splatters around, it smells pretty bad.

No the boots are all tight. I had trouble identifying what the smell was as it’s a bit sweeter smelling , and less than typical hypoid gear oil. I pulled the fill plug in the rear diff and it’s the same smell 

On Subaru Diffs I've pulled there is a vent plug on the top of the diff.

You could always drain and fill it with Amsoil gear lube and see if the smell goes away.

8 hours ago, lmdew said:

On Subaru Diffs I've pulled there is a vent plug on the top of the diff.

You could always drain and fill it with Amsoil gear lube and see if the smell goes away.

I think you may be right. Took a look at a spare differential and there is a plastic plug that comes out pretty easily. Unfortunately it can’t be checked with the differential installed. My mechanic is going to go over the car again anyway also for a new noise it’s making