I'm really sorry. I hope Buttercup is alright. 
Hawks are such a worry. First thing I saw when I went outside this morning was a hawk - tried to get it to leave and it just stared at me, majestic, not the least bit motivated to go anywhere. I left my feathered friends in for several hours and then let them out and watched them like a hawk. But there are always those short spells where I can't stand over them even when home, and yet cooped up in the run for too long they go mad, hen bawking to beat the band to get out. 
I hope somehow your Buttercup comes home. I'm sure you searched the area in case she's injured somewhere....
JJ