[DennisAN]

I come from a background of Japanese cars, but.....

Throttle bodies may have a "plastic" layer installed right where the throttle plate rests when closed. This serves as a "gasket" to ensure complete closing, with idle air supplied from a bypass. It's a common error reported in certain forums to over-clean throttle bodies with harsh chemicals (e.g. carb cleaner) which removes this carefully-manufactured plastic layer. Then the throttle body will forever leak a random amount when the throttle plate is closed. In the US there's a special "throttle body" cleaner sold that's less harsh than carb cleaner, ostensibly to not remove this plastic liner.