Motion alerts in rain, heavy fog/mist, dust, bugs, etc can be tricky to deal with.
The main approach to reducing the false alerts is adjusting the sensitivity of the motion alerts through the available controls (duration, sensitivity, area of interest, alert of people only). Another deployment practice that can help is to use an external IR panel instead of the onboard IR lights. This is especially helpful on the MV52 which I find to be the most sensitive to rain drops, snow, mist, bugs, etc.
For on demand searches I find the people and vehicles filters in Vision Portal do a pretty good job cutting out many of the false positives.
I hope we see more refinement with motion alerts in general.