A small piece felt while disassembling lower case

Hi,

I followed these steps to disassemble lower case (MacBook Pro 17" Models A1151 A1212 A1229 and A1261 Lower Case Replacement). After removing superdrive in Step 13, I closed the lid and flopped the laptop to see the back part. While doing that a small piece felt apart (See images), and I don't know the *#@& where it comes from and where to put it back. Please, I need urgently help, I'm despaired! :(

Thanks in advance

Pete, thanks for the answer.

I still don't understand why it broke, I didn't brute force anything! :(

Do you know where it fits exactly in the attached picture? In the blue or red shape?

Is it possible to stick it, with superglue for instance?

Thanks.