Sensor output intentionally coupled to its power supply net
It's basically like an electret microphone: -



And inside you might find a jfet amplifier like this: -



Note that the 680R and 6k8 in the two diagrams can be any value between 680R and 10k (microphone dependent).

It may not use a jfet amplifier given the low resistance feed due to the 49R9 resistor. Electrets tend to use jfets because of the very high input impedance they have got. More than likely it's a single-transistor amplifier but could be a BJT - it all depends on what the actual sensing transducer is.

In industry, powered sensors tend to use separate pins for outputs but as this is likely to be a simple transistor amplifier there would be no reason to use three pins - it uses a method called phantom power.