Marlene DID NOT go wild!

Like I said on your wall post: I think a plausible reason Marlene didn't go berserk in those episodes (other than that the animators forgot) is that she wasn't aware she was outdoors. In all the other instances she was outside the zoo, she had good reasons to not even realize she was outside:
Roommates: she was focused on either getting rid of or saving the life of her roommate, Rhonda.
Golden Squirrel: not only had she just been kidnapped by the rats, but they were involved in a ridiculously rich treasure hunt.
All-Nighter: The kids were depending on her! It was Kidsmas!

However, in the two times she did go wild, she was almost painfully aware she was outside the zoo doors. Therefore, my hypothesis is that she only goes insane when she KNOWS she is outside the zoo walls.

She's keeping a secret...

Even in the video game, Private asked Marlene why she isn't wild when she was talking to them outside the zoo.

well I don't know but wall she was by skipper she din't go wild. mabey she has feelings for skipper

I guess they forgot... That's common in a series with many episodes.

Preoccupied with other stuff? I'm saying, she had amission on her mind and going feral would not help.

I had noticed that too, with the all Nighter before Christmas. I was thinking WHAT???

But, she was distracted with other stuff, in the episodes in question, therefore, did not go feral.