Any company reviews for some sort of motion monitor?

My sister & I alternate staying with our mom, but sometimes she gets up during the night from her bed & we don't hear her. We're looking for some type of "motion alert" that we could put at her door that would alert us. I see lots of options online, however our local medical equip. company says there are LOTS of problems with these & they stopped carrying them because of that. Anyone have any suggestions that we could look into?