I've noticed this a few times. I have a 2016 Legacy 2.5i premium.

On occasion when I've opened the hood after driving, to say fill the wiper fluid as I did yesterday, I smell a bit of an oil smell. Granted, I had an oil change about 5 weeks ago but I've noticed this before between oil changes, and I got a decent whiff of it late last winter when I didn't open the hood but cleaned out the packed snow from the grille. The air escaped from the engine compartment and I smelled a bit of oil. It's not strong or anything and it doesn't come into the vents while I'm driving. Sometimes after parking it and walking away from the car, I swear I smell a passing whiff of oil, also.

It doesn't appear to be leaking. It doesn't really consume much oil and I've been watching my dipstick. Maybe a teeny bit but not even enough to bring it down to the full line as the Subaru dealership always fills it a bit above the full line.

I've heard something about cam cases in this generation Subaru having some leaking. I'm not sure where that is or how to check it, even if just visually. I have 15 months left on my CPO extended warranty on the powertrain so if something needs addressing, I'd like to tackle it ASAP. I'm wondering if something may have a small leak and it just burns off at the site so it's not actually dripping.

 

Check the PCV Valve, to see if it's stuck open.

I have had a leaking axle boot sling grease onto the exhaust, which can cause a smell without actually dripping anything on the ground. Beyond that, it could be just about anything. IIRC the catalytic converter is right up under the engine on those, so even the slightest drip will end up on a very hot exhaust component and burn off. Will likely have to remove the under tray, and look around with a good flash light to find evidence of the leakage.

Under tray can hide heaps of stuff! It’s worth dropping that off for a look around if you can. 

I doubt the dealership would want to look for any problems for a warranty job, but I could be wrong there. 

Cheers 

Bennie