Looks like a red plastic plug used to plug a hole in something to keep it clean till it was installed. Nothing on your engine that I recall uses a plugged hole like that. Could be from something other than engine that was installed in the engine compartment.
Ayuh,... Looks like a 3/8"- 1/2" port/ pipe plug/ cover... I've got a toolbox full of 'em....
Like Don said,... It's Throw-Away shipping packaging.....
