Hello,
since I have a few sensors that won't report their state anymore (either stuck in hardware, or their events were never received by the hub due to radio/routing issues), I would like to write an app to monitor for events and:
Is there a way to implement 2.1 with sendEvent from an app?
This is a great idea for an app! I would use it. Keep us updated on the progress.
So, this isn't exactly what you want, but Hubitat Safety Monitor and at least one community-created app (EricM's "Device Monitor") have the ability to monitor battery status of sensors and report which ones are below a certain threshold, etc., or (in the case of the latter) which ones haven't been "active" for a certain period of time, often indicative of a dead battery. I mention this only because dead or dying batteries can be be a cause of the very issue you mentioned.
For other causes, I'd suggest improving the health of your mesh network--as you know, messages shouldn't get dropped in the first place, so that's the problem I'd really want to solve. 
Use repeating devices where necessary; ZigBee devices should find the best route automatically, whereas Z-Wave may need a network repair after moving or placing repeating devices. Avoid "known bad" repeaters, which include many ZigBee smart bulbs when paired as ZHA 1.2 devices directly to the Hub (I have all my bulbs on Hue for lots of others reasons, but this one doesn't hurt, either). Placing the hub/stick in a central or other desirable location is also probably best, though repeaters should help regardless.
Another cause could just be a bad sensor. I've had a Dome DMMS1 do this to me, but I think it's because the battery was dying and I moved its parent/routing node, so it was probably no longer able to reach it. I think some Xiaomi users also reported similar problems on at least ST, but I think the most popular community-created Hubitat driver for this resets motion to "inactive" a certain number of seconds after the last "active" message from the sensor (I think that's the only kind it sends), so that probably doesn't happen anymore. I've had bad luck with cheap Z-Wave motion sensors, though not usually this particular problem.
I do agree that network issues should be solved beforehand, but I still need something able to detect and force inactivation of a stuck device.
Yes, I had a stuck motion detector, that had been stuck on active for 12 hours. And even though I was walking in and out of the room, it wasn't sending active or inactive. It's the Bosch motion detector. I couldn't think of any way to reset it.
I looked at Rule machine, and couldn't find anything. The only thing that I could do to get it unstuck was set it to a simulated motion detector, hit "inactive", then change it back.
I imagine it wouldn't be too hard add buttons for active/inactive to the inbuilt Bosch Motion Detector driver? @csteele @mike.maxwell
Curious. I have added Zigbee plugs to enhance my network. Since doing so, no issues. I also have an app that updates all my sensors every half hour so I can monitor “stuck” state. I wonder if “waking” them has an effect?
I've started having this same issue. Thing worked for 2 months and then just hung up in Active. Thanks for the tip on changing the device type. I had been removing and re-adding mine 
"Stuck" is a failure of some sort, and @bravenel and @mike.maxwell are usually against band-aids. Shockingly, they would rather spend hours today to fix it, for a quiet future.
In my case, I've seen this BUT it is in my head as a "display error" in that it doesn't seem to affect a Rule or a Dashboard.
"inactive" seems to be the word for "this device is expected to wake up every hour, but it hasn't... yet."
I getting in touch with Bosch support to see if they have known issues...
Anyone have any luck with this issue? I've noticed a couple of posts over in the ST forums with folks having the same problem with no resolution. Not sure it matters but all of my Bosch sensors came from MyDigitalDiscount.
I can confirm I still have issues, I'm in contact with Bosch but they are blaming the hub.
My sensors were from MyDigitalDiscount too.
I wonder if
a) mdd has a batch of faulty sensors
b) all Bosch sensors are faulty
c) they have different revisions
Possibly 1 + a=exhibit 1a 
That's what I was wondering. I sent an email to MyDigitalDiscount yesterday. Maybe I'll hear back from them? :)
I noticed today that these were still alive on the zigbee network and reporting temp, but they were not reporting movement at all. I've just opened mine (red light now flashing on movement, just temporarily) and will monitor them over the next few days.
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