I believe it would be better to replace "behind" with "after" to be consistent with the use of "before" in the following paragraph :
J'ai regardé la fille. la fille is the object of ai regardé but it's behind, so no agreement.I watched the girl.
-> Je l'ai regardée. l' replaces la fille -feminine/singular- and it's before the verb, so agreement.
I watched her.
