Last night I hosted a Terrorist Hunt and, during one mission, I was holding the back button to view the map. As I was doing this, a terrorist killed me. The map and unit locations disappeared, but the tinted square and legend remained while I was in observation mode. After starting the next match (on the same level) my mini map was tucked away as if I was playing single-player. I could still view the full map by holding the back button, but the upper-right hand corner of the screen was empty. I played match after match like this and it stayed hidden. I happen to like having that handy, so, on one match, before the last terrorist on the map was killed, I kept the full map open by holding the back button just before and during the "Mission Success" announcement. On the following match, the mini map returned.
I read somewhere on this forum someone's complaint about not being able to turn off the mini map in multiplayer. I haven't tried this in PVP multiplayer, but I suspect doing this might have the same effect.