Mystery Part

Look at your vacuum hoses

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is it rubber or hard plastic? plastic maybe vacuum related
rubber maybe frame related

 

Looks like a check valve for a vacuum line. Likely got dropped and lost during some service.

 

It's rubber. It's a filler plug for vacuum hoses.

I too found a couple in my car and I could not find where it belongs.

I can only assume its extra parts stuff from the factory left in the engine bay.

 

Solid rubber plugs like this can also be used to secure two items together.
This picture is from a Porsche 928 in fender

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