I sent Piper to my settlement at that Drive in Movie place, a pretty secure location since Sanctuary was full. When I went back to pick her up she was nowhere to be found. I rang the bell to get everyone there to find her, searched the entire area, checked all my settlements, checked her place back in Diamond City, and I still can't find that woman. Any ideas on how to find her?

And I know she was there cause I saw her once before she dissapeared at the Drive Thru. So do I have to put sirens at all my settlements and hope I find her?

Try waiting/sleeping for 12 hrs. or so where she is supposed to be...that has worked for me every time I've lost a companion.

Thanks for the tip

Did not work

I lost dog last night. I fast traveled somewhere and there he was.

If all else fails, try looking where you first met her. Sometimes NPCs go back to their default locations when there AI schedules go awry.