I am finding that enemies (MP) seem to be able to know exactly where I am on the map and they just pop round the corner and kill me before I get a chance to do anything.
How are they doing this? Is it clearly shown on the minimap where everybody is, or are these guys using a hack or something?
I have tried looking at the minimap to locate other players, but have had no success in determining where they are.
Is there a larger map that can be pulled up?

Either you stayed stationary without equipping blindspot module when enemy have max radar support, or you may just run into some wall-hacker.

Your exact position is shown on the minimap when an enemy's team is using Maximum Radar.

the radar updates every second... so even though ur at the point where an enemy is supposed to be.. he might already have moved off to another location in that second...

thus, radar provides approximate location...

when ur near, switch to ur ears and listen to footsteps, weapon sounds, shuffling... and pinpoint its location using stereo sound

Just a little hint which everyone problably knows:

Blind spot on the radar = the target is above or under your level...
Filled spot on the radar = the target is at your level ready to be knocked down...