Remember that the ECM's pulse travels through walls, so you don't need to get into a security room to shut it down. It's best to place the ECM Jammer near the security room, if you place it at all. If you're in a place with a security room (such as any of the Bank Heist missions), and your ECM Jammer doesn't affect the guard in it, then he'll call out when sees a camera go down.
You'll want to be a little careful with the Jammer depending on the mission. If you take down a guard, then place the jammer, you'll need to interact with the body immediately as usual. Further, once the Jammer finishes working, the pager will become active again, and you'll have to interact with the guard's corpse as normal or the police will be alerted. The Jammer also shuts down the pagers of downed guards if you placed it before shooting the guard.
