the post you've now deleted said that your grandparents were your legal guardians.

either way, it's not entirely relevant. you had some people that you live with who you knew would expect you to come home at X time; you didn't come home at X time, and they (quite reasonably) believed that you were missing.

"did you have permission?" and "are you missing?" are two completely different questions. evidently not a single person told them that you wouldn't be coming home that night, and it was 100% reasonable of them to be pretty dang worried. if your post was "i gave my grandparents a heads up i wouldn't be coming home because i'm staying with my boyfriend, and i had permission from my mother to stay with him, and they called the cops on me / freaked out / etc.," my opinion on the situation would be 180 degrees different.

i also live with someone who loves and cares about me. if i'm going to be out later than she's expecting me home, i tell her so that she does not think i've gone missing. i also tell her where i'm going to be so that if i do go missing, she is able to tell authorities my last known whereabouts.