Interfacing incompletely documented logic signal where resistor affects output voltage level
You should try loading the output with something like a 10K resistor, first to ground then to +12V and see what happens. One way or the other you will likely see a reliable logic change, the opposite way you'll see no change in voltage.  

Sounds like this might be an industrial standard output that can only source or sink current. If you don't put an appropriate pull-up or pull-down resistor on there, you'll be depending on leakage to give you the proper logic state. Chances are that will not be reliable.