Mylar? Does that contain metal or not?
Now, this is quite an interesting trick going on here. Principally, indeed the material is a plastic, one that is related to polyethlene terphthalate. However, there are “metalized” versions where a thin film metal (aluminum) additive is placed within to provide 99% reflectivity and a shiny metallic effect.
So, the point here is that although there still is metal, it is dramatically less metal that aluminum foil. So, your metal detector might either not detect such materials as metal or only barely detect the metal within them.
20180726/https://en.wikipedia.org/wiki/Mylar