hard to tell, i don't see anything obvious, all that rust makes it look the same, LOL. i'm from maryland and live in the rust belt so i'm allowed to poke! i worked in greenbelt, dad works in Gaithersburg. you can tighten the two remaining lug nuts and drive it just fine...to get it somewhere.
in some cases there it seems probably that the rear rotor is causing the lug nuts to shear off. presumably they aren't perfectly flat and this stresses the lugs over time. mine is currently shearing off as well - passengers side rear just like yours 2003 Outback Sedan. I have 2 sheared off at the moment and have been for months. so i take this to mean two things:
1. i would replace the two existing studs as well while you're at it - they're only a couple bucks each and it won't take any more appreciable time to replace them. if those three were fatigued to failure, the other two were probably fatigued as well.
2. you might want to consider installing a new rotor while it's all apart - again cost of rotor is about all it *should* be depending how honest the mechanics are.
I'll mention the rotor to the mechanic. He's replacing all 5 lugs (not additional labor, $22 more parts). The other 3 tires are fine.
This sides braking was repaired 3 months ago for a groaning sound. He said if it was installed a little cockeyed, not in that star tightening pattern, it can get hung up on one lug, seem tight but not really be. I'm not sure I understood the "hung up" comment. All makes sense though. And with a few people posting -- seems to be a thing with Subarus.
This car was my parents in upstate NY, so MD is rust-free by comparison . So tease away.