Room 12500

[Norris]

Hi folks,
I recently started playing these custom mansions (what a great addition to the game btw) and I stumbled upon this one and so far I like it a lot.

But, not surprisingly, I am stuck.

I have made it past the first saving point and the first room after that but then I get stuck - it is room 12500.

I can't find the green key.

I have tried almost everything and am starting to think I should have brought the key with me from the room before.

Any hints?

thanks,
Norris