Cat knocking off this cap. Anyone know where it goes?

This cap was found under my 4runner on a few occasions. Just so happen around service time and had service pop it back on. Did not ask them where it goes. This happened 4 times in the last 10 years. Twice on my prior 2006 4runner and twice on my current 2016 4runner.

I'm thinking it is from the engine bay, as usually that is where cats like to hang. Know it is a cat knocking it off by viewing security cameras.

Thanks for your help.

 

Get under the 4Runner, look at the bottom of the tub where you feet and stuff go, near the door sill to the edge. That looks like a body plug that would be used to drain water, run cable.

 

Thanks for the quick reply. Will check it out later today.

 

What fajitas said. It's a hole plug.