How do I get to Riddler's second hostage in the Enigma Conundrum side mission?

I've rescued Riddler's first hostage from the courthouse, and gone to the location of the second hostage, outside the Black Canary. However, I've search all over the building and can't find any doors or other entrances. How do I get in the building with the second hostage? Or do I have to go somewhere else?

Solution 1:

Look for a green question mark on the wall. You should be able to walk up to it and break through the wall to reveal a door hidden behind it.

Solution 2:

The spot shows up on your mission quest long before you can actually click to get in, at least for me. I had to find about ten more trophies before the question mark showed up... And the question mark shows up on detective mode not in regular mode.