If you replay the video stream in SmartPSS (assuming you are doing 24/7 recording), you can inspect the "green" walking squares/blocks when the fire truck passes by. If you don't see any green stuff, the camera simply did not detect anything. If it did draw them, follow the directions on where the blobs were going towards. If they don't "cross" a tripwire/intrusion box, there won't be an IVS trigger. In my personal experience: cars with headlights (and especially the ones with flashlights) will create much larger "target" size than anticipated.
It's testing out, tweaking, testing out, tweaking again, pulling some hair out, tweaking and doing all over again ;-)
CC