The 0–5V ADC port leaks a small amount of current
Although the ground for the 3-wire connection is disconnected unless the sensors are active, the 10K+15K resistor pair to reduce the signal to 0-2V input for the ADC is always connected to ground. As such, there is a low current path to ground through the sensor signal line (≤500µA) via the 10K resistor.
The bottom of the 10K resistor should also feed into the the low side switch FET (Q1) so the current path is blocked unless activated; instead of directly to ground