ACE Chemicals is an established part of the DCEU, with this specific location appearing in Suicide Squad. Billy Batson practiced his superpowers at another ACE Chemicals plant in Shazam! as well. Later in the film, when Harley needs a place to arrange a meet, she chooses the Joker's turf - Amusement Mile. An abandoned amusement park in North Gotham, this is traditionally the Joker's base of operations, most famous for its role in The Killing Joke. It seems the Joker has decided to leave that for Harley, because he's taken all his equipment out and left a crate packed with Harley's few belongings. Look carefully, and you'll spot "HAHAHAHAHAH" scrawled on one wall in a similar style to the Joker's laugh in The Dark Knight Returns. It also resembles the distinctive tattoos on Leto's Joker.