You know what’s great about motion sensors? They are very, very cheap to build yourself. All it takes is an esp8266 module like the Wemos D1 Mini, some PIR sensors, and the ESP Easy firmware and you can have a bunch up and running in a few minutes. I haven’t DIYed a battery powered one yet, but there are plenty of great Z-Wave ones available.
Once you have a couple of motion sensors in your smart home, you can have Home Assistant track the last place it saw motion. This is a useful bit of info – useful as a condition for your automations, or as an input for a bayesian binary sensor. See below for YAML to create a meta-motion sensor with a history.