What is this part I found loose?

It's a standard plastic clip used to hold interior trim panels, etc. Don't know it's actual function in the Model S but it's not a cause for concern. One of the legs is broken and it may have been a discard when the car was built.

 

I found a white one on my rear floor too. I asked the service guy and he said it was just something that held something temporarily together during the manufacturing process.

 

They use a combination of those and really nice metal clips for the panels on the interior that I have removed. like on the plastic panels you step over when entering the car. they have a couple of those in there + metal clips

 

Don't listen to these guys. This is a very serious piece of equipment. In fact, the car is probably junk right now and you should drop it off at my place before you get yourself hurt.

 

Oh my GOD!! "Something is wrong with the left phalange!"

 

And if you have range anxiety about getting to Krugerrand's house, you can leave it with me. I have a bunch of similar clips in my garage that my wife and son have been trying to get me to get rid of somehow.

 

Nobody uses the left one; it's all about the middle one...but, anyway...

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Hey, I'm not afraid to go outside and arm wrestle. *rolling up sleeves, cracking knuckes, and putting on my big girl panties*