Alcohol impairment can be very messy. I know someone who had 1 and a half drinks, had no speech slurring or obvious signs of being black-out drunk, but did not remember a third of the night the next day because she’s a lightweight that’s drank a single-digit number of times in her life (she’d also eaten very little that day and had some recent sleep debt). I would not have guessed she was blackout drunk, she was just talking about her problems and was articulate the entire time I saw her. If I didn’t know she had been drinking, I can’t think of how I would’ve known short of some kind of specific motor function test that’s made to suss that out/harder than just sitting around, talking, and occasionally going to the restroom. She just looked a little tired. When we spoke later, she didn’t remember basically anything after a certain point (a couple of hours’ worth) and wondered if some vague flashes of memories had been dreams. It was rather surprising to me.