A what is this thread?

[lockedup5]

As some may know by my build thread I'm basically building a camper. I'm kinda stumped on what something is.
I traced a wire from the LP gas detector thinking it was going to a solenoid valve that stops the flow of gas if LP is detected. What I found is this little thing that the gas line goes through but it slides on the gas line.
Anyone know what it is?
One wire is 12v hot from the detector and the other I'm assuming is ground.

 

[snappy]

Its a propane cut off.... If it detects a leak it shuts the gas off. Had to replace the one on my truck a few years back

 

[snappy]

As in a external sniffer...

 

[lockedup5]

I'm just wondering how it cuts it off unless it just cuts it off in the furnace itself. I don't see a solenoid anywhere the LP lines are connected to. My LP detector is inside.