I was wondering if anyone else was encountering this same problem I was having. I assigned one of my companions, Preston Garvey to be exact, to use one of my power armors and I dismissed him back to sanctuary hills to help out. When I fast traveled back to sanctuary hills I see that he was out of his power suit and it was nowhere to be found. This is really disheartening because it seems I won't even be able to go back to a previous auto-save/manual save to try and fix this because at this point. So I'm posting here to see if anyone else has encountered this problem and to see if we could get a fix this problem in the future.

Look around sanctuary, it may not be close by the fast-travel landing spot. I've found NPC power armor out in the field where I grow my crops. Not sure why it happens, but there you have it.

Ok thank you I'll try to look around his usual patrol routes I just hope it didn't disappear.

Update: Ok I managed to find the armor now I just followed Preston around on his normal routes and I managed to find it in one of the houses. I'm not entirely sure why he left it in there. It seems like he just abandons the armor wherever it is when he is going to sleep. Would be possible to tell the npc that we assign to a power armor to go back into that power armor once they are done sleeping?