I've found a few people in my fort completely drained of blood. Does this mean that I have a vampire among my dwarves or is there another way for this to happen like bleeding out or just remaining dead for a while?
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.
This is old but others might stumble upon it so I figure I will answer it;
Its true that "completely drained of blood" is signature of a vampire infestation its strictly not true that only vampires drain blood. Certain bloodsucking creatures does too, and if you have a forgotten beast who use one of them, say, a tick, as a base template they can drink blood too, potentially all of it. I have not personally seen a dwarf sucked dry this way tough, so I don't know how or if at all the game informs you of this. But you would probably know if a bloodsucking forgotten beast is in your fortress so you would likely not be confusing the two.
Vampires are the only thing in the world that can driand ALL of your blood. Nothing else can because they will not drain all of it but most of it to the point of death.