None that I've heard of. Buffy said she walked out of a room for half an hour, and when she came back, her friend was gone. Some people write letters, in case it happens. I got one from a friend, not too long ago.

[He stops, and cannot continue this babble. It's finally hit home for Horatio, and that's a scary thing, scary because he knows the panic that comes with it himself. Wondering if it would be this heartbeat or the next, just like on his deathbed.]