It tracks your devices, and it's proprietary software. Furthermore, from what I understand about the PC software, it runs at the kernel level (ring 0). I don't believe it can do that with an Android or iPhone app, though.

Either way, if being tracked on a whim is within your threat model, you should assume this is spying on you. Most modern phones already have some kind of "Find My Device" tracking, so this seems like you're just adding an extra pair of eyes.