Can people have all their blood drained without vampires?

No they can not. I'm pretty sure this message is a dead giveaway of the existence of a vampire. I think even dwarves killed by giant mosquitos aren't "completely drained of blood", just dead. Only vampires are so fastidious.

No. Finding a body "completely drained of blood" is the game's way of saying "Killed by a vampire".

Finding out which of your dorfs is the bloodsucker is going to be tricky - I recommend checking this section of the wiki for tips.

Most of the vampires I've managed to detect gave themselves away by having ridiculously long lists of places they're former members of -- taking a cursory look at new immigrants' histories will go a long way toward being able to tag them as potential vampires early on.