I just thought of this and its not my best idea but I think it would be a good way to get more people into special weapons because of its low credit.
I am thinking about a 2 or 3 ammo deployable sensor that you could drop in hallways or doorways that would alert you if someone passed it. Since every good map has more than one way to get into almost every room this wouldn't be a tool for ultimate camping. On top of that you wouldn't know if it was an ally or not that passed the sensor.
I was thinking roughly 3 credits if the sensor continuously told you when people passed an closer to 1 if it only told you for a certain amount of time or passes.
Pros: Good way to know if someone went into get the hostages. Low cost credits.
Cons: Not extremely useful and may or may not be possible with Counter Strike.
Sorry for the undescriptive post, I'm tired and I didn't put much thought into it.