Detected baby 'moving' when she was not even in the cot!!!

My grandaughter had this monitor at the recomendation of a Mothercare employee. She spent the night at mine and in the morning when she woke up I forgot to turn it off. She was sitting the other side of the room in my bed and the monitor was still registering her as moving!!!!!!!!! It did not alarm and continued to show the light claiming she was moving. I turned it off ten minutes after this!!
My first son died of cot death so I wasn;t ahppy relying on movement monitors so I invested in a SP02 oximeter from China. My daughter doesn't like this monitor so i just use it at mine but I was so greatful that I had it on my grandaughter that night as well as this totally inadequate so called monitor. Thankfully she was fine but that was no thanks to this monitor!!! Mothercare have swapped it for another one now.