You can only get it if you hired Ivy
![Sneaky :sneaky: :sneaky:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Put her in charge of the Bordello and place Batgirl inside. There are several such little extra scenes but only you know what you seen or haven't. Use the walkthrough.
You are still missing something, even if you think you don't but you do. This is an automatic event. Two possible explanation for it not to trigger:
1) One of the unknown prerequisites haven't been met. Noone knows which are those, we all get by through trial and error, the guides were built through it as well. Use the guide and instead of assuming you have done everything assume that you haven't. So try to do everything all over again, otherwise you are just stuck in a loop.
2) Your save is broken and your trigger is messed up. Nothing to be done. You can try and through the cheat menu re-lock then un-lock them again, see if that helps.