If your daughter lives at home, whether 30 or 18, she has a responsibility to check in with you to a degree. She is still in school, so she should have a curfew by all means.
I think for an 18 year old 10pm on school nights and 11:30 on weekends is more then reasonable. No 18 year old needs to be out after midnight. Most cities have curfews for kids in school that are even more rigid. On school nights she should be at home, studying, doing homework and helping out at home. She sounds like a good kid, maybe she did fall asleep, however remembering being 18 I know too that they are totally capable of stretching the truth to suit the situation!
I say there has to be consequences if she doesn't check in.
I say she should have her own cellphone she pays for herself and is to answer it if it rings, set an alarm for when her curfew is up.
My concern is why is she in a position to fall asleep anyway? I mean that implies she was laying down somewhere so I would question that if she was out playing games. If she changed where she was going, you should have been called.
At her age I was allowed to set my own curfew (within reason) on vacation from school, during the summer months and weekends, however if I said I would be at a specific place I had to be. If I said I would be home at midnight, I would be or my priviledges were taken away. I never pushed my boundaries, I felt I had some say so in the matter but at the same time my mom wasn't worried about me either. On school nights I couldn't go out until homework was done and had to be home by 10pm.
If she lives under your roof you have to be able to lay down rules and expect them to be enforced. You however have to be the enforcer, which means you won't be popular sometimes, but being consistent, clear and following through with consequences, she will appreciate it one day! Promise!