Re: Passive sensors?
Yes, that's just the sort of applications these things could be used in. No wires, no batteries, simply drop-n-go.
However... most security solutions aren't simply passive until activated. They will normally wake-up occasionally and send status signals to prove they are working, haven't been tampered with, and even just to prove they are still there, haven't been stolen! Most importantly to let the system know, as best as can be done, that it will work as expected when required to.
The last thing anyone wants in a safety or business critical application is something not working when it's needed to work. There is also an issue of guaranteeing delivery; a sensor would normally keep sending an activation in case the receiver does not get it. Not sure how that would work if there's only one activation and no power left for sending a subsequent alert packet.
I would expect most serious safety or business critical systems to be hard-wired, non-wireless, just to overcome the problems wireless and/or battery powering presents. I can see there might be a use in less critical applications such as self-powered TV-style remote controls and similar
The biggest issue perhaps is cost and it's questionable whether they are actually any better than the current range of battery powered but long-life sensors and controllers.