My guess would be the entire complex is wired from a central location and the filter that's normally in place at each location is set up there instead. It's also set up to make all the channels you saw available should someone install a cable box (rented from Comcast, of course). That way, Comcast saves money because they don't need to contract a technician to do the wiring/filter installation. You can just pick up and install a cable box (from them) and your set.
Since the box your friend brought over was probably already activated and recognized by your local Comcast office, it decrypted the channels for you (this is assuming you are a Comcast customer). However, you should note that a digital converter box is NOT a Comcast cable box. It will convert the digital channels to analog, but won't unscramble any channels (only a Comcast activated cable box will do this). So no, buying a converter box won't do you any good.
-Wolf sends