The plaster is probably on metal lath or the walls are masonry (or both) but you haven't told us anything about the building.

Is there an outlet on the wall?

ended up just drilling straight through and into the bedroom.

If you didn't use a masonry drill, that would indicate it's a conventional wood partition wall. Why not just ask the super?

If this is a large building, it could have all sorts of pipes or electrical wires in the area you are working. I personally would open up a section to see whats up.

There are a couple of large apartment buildings in my city from the late 50s early 60s that are beam and column construction, and none of the interior partitions have studs. There are some units that have masonry walls in between, others have this strange "fireproof" construction where strips of drywall were used as "studs" and two layers of drywall were glued to each side. Some of the partitions within the units are layers of drywall or some kind of panel and are less than 2" thick.

Some walls have conventional outlets and some are in surface mounted boxes. Most people who renovate these units now tear out the partitions and start from scratch since rewiring or anything else is a nightmare.