Why am I seeing only 30v across a light?
It is phantom voltage, which is a small voltage induced (capacitively coupled actually) from adjacent power lines.  (needless to say, those power lines need to be on.)  It's sorta like picking up radio stations except the adjacent power line is much closer, so the voltage is much higher. 

On a 3-way switch, you have two parallel traveler wires in the same cable or conduit.  The 3/4-way switch complex changes which of those travelers is energized, and whether the energized traveler connects to the light.  Ideal conditions for phantom voltage being induced in the un-energized traveler.  So if the switch is off, you will see this voltage at the lamp socket.  

That must be what you are seeing.  

The voltage has no real energy behind it.  It can't do useful work, and if you connect even the smallest load, the voltage will disappear.  It can only be seen by very sensitive voltmeters such as the common commodity DVM.