When these mercenaries were first added they shared a spawn across zones. Thus you could never really know which zone they were in. This caused some issues if noone was in one of the zones so we pulled it down to them only spawning in one zone. Each of them have well over 20 different spawn points across the entire zones not just two or three as some have reported.
There was an issue a week or so back where they were spawning much faster than they were supposed to but this has been fixed and they are back to spawning at the rate we wanted them to.
If you are interested in why he is called a Voodoo doll and not a moppet you can read the dialog he gives and it might give clues to exactly what happened with Kenny (Not Billy)