It does and it doesn’t detect motion. It detects a moving heat source, such as a person, or animal, or anything that moves, and is a different temperature to the background, against which it’s compared.
Sometimes your wind chimes will go undetected, because they are at the same temperature as the ground. Other times they may be detected, because they are warm, and the ground remains cool, or vice versa, as one takes longer than the other to change temperature. The plastic sheet, I could imagine, will go undetected more often, because it will always be at background temperature, because there will be no lag in its temperature change, because it is so thin.
Both of those things might be causing your triggers, and both could behave differently at night, but specially the chimes. I can only suggest trying taking them down and see what happens.
EDIT: Austin beat me to it.