Responded to the wrong comment in the thread because I'm smart like that, lol.

Just wanted to add onto your points - Cisco worked at the library, so he probably spent a lot of time there, which would make it his "HQ" of sorts. There's a season 1 deleted scene where Darlene uses books in the library to relay messages to who I assume are other fsociety members, or maybe she's infiltrating comms between Dark Army members...but it's hard to tell. I think it's most likely the latter because in season 2 we see that Cisco arranges a meeting with the Dark Army there (The scene where Xun sticks a needle under his fingernail), so I think the library can be seen as a DA meetup spot of sorts.

It's implied Leon was purposely put in jail by the Dark Army to keep an eye on Elliot, so the prison cafeteria - or the "diner" in Elliot's mind - is his ad hoc HQ because it's probably an easy way for him to communicate with other inmates and other possible DA undercover agents.