No, this is not ok.  At 18, she may be considered an "adult" but isn't mature enough to know she has a long way to go.  Adults who love and care for one another are courteous to one another.  That means considering the other person's feelings and giving a quick call to let you know her plans.   She would likely be alarmed if you left for a weekend and didn't let her know where you were going and when you would return home.  Also, she must understand it is a safety issue.  Women are vulnerable and if something would happen to her, I'm sure she'd rather you take action quickly rather than wait.  It also seems you may need to have a frank discussion about other issues, - is it possible she is engaging in behaviors that will put her at risk for pregnancy, disease or a violent crime?