This was the issue that was solved for me by mmWave sensors. Having someone sitting very still would fool regular motion sensors (Iris v2/v3), but the mmWave sensors do a much better job of sensing presence w/out motion. They are relatively new and not as completely baked as traditional motion sensors, but can be worth some effort to set up.
Look here:
...and here:
The Linptech (and some others, IIRC) has a known issue that affects some users where the device may stop reporting. Solved by some of us via regular device reboots (device is plugged into a smart outlet or smart plug).
