Why does voltage still exists when a circuit is broken by a 3-way switch?
Do some research on capacitive coupling, and Inductive coupling. This should explain why you are seeing voltage on the "dead" traveler.  

If you load up the traveler that is not carrying current; with a test light for example, you'll likely see this voltage go away. 

Working in a residential setting, this voltage is likely harmless. Phantom voltages, however, can be lethal when working with distribution lines and other high voltage situations.

You may still want to have a licensed electrician come out and take a look, just to be sure this is actually the case.