Probably metal studs

 

You could also be hitting plumbing drain/ vent or supply line. Is there a bathroom or kitchen or the other side of this wall?

 

when you said high rise apartment I was instantly thinking of metal studs as well. I would try to find a stud on another wall and "test" it too. If you can't make it into that one either its probably metal studs.
Have any large magnets?

 

When I heard Hi rise apartment, I'm thinking concrete.

 

The thought occurred to me that it might be metal studs - I'm going to check with the building management to see if this is true. If they are metal studs, any suggestions on how I might attach a TV mount to the wall?

As for the wall itself - no, it's not adjacent to the bathroom or the kitchen. It's a bedroom wall that is adjacent to the living room, so it seems unlikely that there are drain pipes passing through there.

 

I'm going to check with the building management to see if this is true.
If they are metal studs, any suggestions on how I might attach a TV mount to the wall?

Yes; drop back ten and call the management or LANDLORD
Ask them the what and how

Odds are they'll do something for you rather than have to repair later

 

Sounds like concrete. You can tap into the holes you made with a nail and see if you hear metal.