when you have plant the bomb on the fuelling station
No, that`s wrong. In multiplayer you get it if you make 15 kills with a silenced weapon. Pick a weapon, make enough kills to unlock the silencer, then make 15 kills with it equipped. The heartbeat sensor is unlocked after that. In singleplayer, you get it from the beginning in the mission cliffhanger. Just follow the instructions on the screen.