Lose wire next at gas line?

So "m outside weed wacking ... and when I get to where my gas line comes in fro my underground propane tank, I see this lose wire on the ground. It's coming out of the ground with the gas line...

I don't ever remember this line before, and it doesn't look like it gets connected to anything on the line .. It can't be a line for a ground...

What I'm thinking is that's it's some kind of trace wire, but that doesn't make sense unless the gas line, under ground, isn't metal. Is it common to use a non metallic line when buried? Just seems strange to me...

If it's not a trace wire, what can it be?

Thanks!