A passive sensor is when you have set a limit of having, say, three active sensors on a subnet but you have installed five sensors.
Two Sensors in that scenario will become 'passive'. In other words ready to take over if one of the active sensors stops for any reason.
You need to look closely at how many sensors you have deployed and which networks they cover.
(default = 2 active if I recall correctly).
Bear in mind that a DHCP-enabled sensor still covers its own local network too in the same way as a non-DHCP-enabled sensor.
Start by investigating the sensors on the network the DHCP sensor is locally attached to.
Thank you very much, Rob.