This is hard to answer, because it really these days has nothing to do with trusting them sometimes, it's trusting the sicko's that are out there.........
I'm not sure I would let my son bike home at all at night........it's not safe even if you are 40!!!!
I think they should at least check in or call when they are leaving if you are waiting up and they know that you are..........
I know if something is going to happen, you probably can't do anything about it, but I'm like you........someone needs to be up.........it's just nature I think...........
If you feel they are responsible kids, then you can't really stifle them much, although at 16 you should have more control than at 18.......talk to them, tell them your concerns if you haven't.......ask them to call if you know you are going to be up.......and then pray..............You can't be there forever, and believe me, I know.......so try to get some sleep even if it's on the couch!!
Good luck and take care.