You definitely want the police to take every missing persons report seriously, which is why it is deadly serious your grandparents do not abuse the system to try and control you. Establishing a "pattern" could result in the one time they finally do report you missing when it matters getting ignored because its "yet another runaway report from grandma" followed up with "shes an adult now she can do what she wants" and you're actually lost or missing.

Did you tell them you may not be coming home that night? If so, totally unnecessary. If not, what they did was a bit extreme but not totally unwarranted.

Remember. Boyfriend/girlfriend also includes the word "friend" so "crashing at a friend's house since they live closer to work" is an accurate statement.