Is one of my multimeters wrong?
The sensor output is &quot;open&quot; when there is no movement, so it will not pull the output line actively to ground (you have to do this with a pull-down resistor, e.g. 100k to ground).
Possibly the green meter has a lower input impedance and pulls the output line low, while the yellow meter does not. Try adding the additional pull-down resistor, but keep in mind that the output current can not exceed 100µA. So something around 100 kOhm will be alright.