How do I hint at something traumatic (like rape) without ever actually writing the actual scene of it happening?
I saw this handled in a movie, (It has been a while, so I can't remember which one) but you can just skip it. You have what went before and what happened afterwards, and the shock of the transition to jar the reader. In the movie the script called for the wife to be killed in a car wreck, but the director just filmed her and her husband chatting in the car and then her being pushed down the hall to the OR. This is basically a tell don't show (not the usual recommendation of show don't tell) and to pull it off it must be a traumatic event, you have to use show don't tell everywhere else to make the brief tell standout and shock, and you have to actually tell what happened.
As the party wound down I made a point to say goodbye to Alice, "I enjoyed seeing you. You have my number, call me. If I can't see your face at least let me hear your voice." She left with a brief chuckle.
As I was policing up the last of the trash, the phone rang. it was Alice. "I'm downstairs. I need a ride to the hospital. I was raped."
. . . skip . . .
As we pulled out of the parking lot, I asked, " Do you want to talk about it."