French language Q&A Forum

Personally I din't like this exercise at all

In this exercise, which asked to conjugate verbs in Plus-que-parfait, I wrote the following sentence: Marc lui avait souri et Gilles avait deviné tout de suite que Marc avait capturé son âme!  My « avait capturé » was marked down and corrected to be « avaient capturé ».  I cannot understand why a 3rd person plural conjugation is being used here instead of singular since the sentence talks about one person, Marc, who caught/captured Gilles’s soul.

During a quiz, the question posed was, 

Vous _______________ dans le placard. 

I conjugated it as Vous êtes cachés but it marked it as wrong. Is there a distinction when the subject is to one person? I am a bit confused. 

Surely it is le mien? Please explain