Quitter - transitive verb - but direct or indirect?

Question

On a recent quiz - "Nous quittons Londres." - was marked correct, while, "Nous quittons de Londres." Was marked incorrect. -- While the lesson was clear as to why "Nous partons de Londres" was the only correct answer, it was unclear on the distinction for the verb quitter. When I read other grammatically correct phrases as " Il a quitté de son plein gré." & "Il a quitté de son poste," where quitter is used as a direct transitive verb I get more confused. What makes the only correct usage in this case -- "Nous quittons Londres" - indirect??

Merci beaucoup!

Accepted Answer

Hi Michael, 
Just to add to Alan's excellent answer.
Il a quitté de son poste is incorrect 
Il a quitté Londres = He has left London
Il a quitté son poste = He has left his job
Il a quitté sa femme  = He has left his wife
De son plein gré only describes the manner in which he left but you cannot say quitter de.
Hope this helps!

Answer

"De son plein gré" means "of his own free will", so it's not the object of the verb quitter. Although the lesson says that quitter has to have an object, I think it's acceptable if the object is understood. In full it would be "il a quitté son poste de son plein gré" .Are you sure that "Il a quitté de son poste" is correct? It gets a few hits on google, but there are many more for "Il a quitté son poste".