if getting caught by the attendant is a problem, what is the 'official' purpose of the booths? You don't need them in a regular bookstore if you want to browse the materials.
i don't know about elsewhere, but in Oregon, having sex in a locked booth is legal because the law here treats it as "private" consensual sex, just as if you had a hotel room. One of the local ABS that is most popular shares a parking lot with the State Police lol. Times have changed.
As far as signals go, i see visiting an ABS as like going to another country with lots of mixed cultures and different languages. There are some pretty commonly used signals, like putting your finger through a GH generally signals you want cock, usually to suck. i'm bottom and i'd say easily half the guys i offer ass too only want a bj. my guess is most are on the DL and simply afraid to go beyond certain limits. Heck, look at the comments here and several Zity kinksters express fear or hesitance. On the other hand, the other half seem like they cannot believe their good luck at being able to fuck and usually come very quickly. i've had more than a few guys ask through a GH if they could come into my booth cause it's easier to fuck that way. Some leave their door open hoping for someone to open it. Some go around looking for open doors. Mostly i think it depends on who's there. i think about half are regulars who know the ropes, the other half are infrequent or newbies who are both shy and unsure about what they are doing. Reality is always different than the fantasy that brought them there.