When I bought my 2003 S-Type 4.2 back in March I found this plug in the
glove box. Looked all over and around the engine and cannot find any place that is missing a plug.

It has 2 rubber O-rings for seals.

 

What size is it?

I can't think of anywhere on the car. With no threads it can't be anywhere pressurised, yet has seals. Dunno what that leaves!

 

Its 3/4"

 

Under your spare tire and connected to your jack is an eye for towing. It goes in a fitting on the front and back of the car. My car has a plug in each of them and that may be where it goes. It is just a protector for the threads. That is all I can come up with.