XBee DIO Adapter Input Voltages a Problem?

I purchased an XBee digital IO Adapter and am using terminals 3&4 as sinks for pulling low 12V through a 500ohm resistor (one circuit on each terminal). When the DIO is “off”, some current still flows into the DIO from each of the 12V, 500ohm circuits (about 0.1ma into each terminal); when it’s on, it grounds the circuits effectively. Why doesn’t the current stop when the DIO output is off? The spec says the adapter terminals can go as high as 30V with 1.8A sink ability. I’m not going over 12V and draw a max of 24mA. What is going wrong (a DIO adapter schematic would be helpful)?