Can someone recommend a motion detector that works from behind glass, or recommend what I should be searching for?
Oh, this could cause some excitement:http://www.parallax.com/Store/Sensors/ObjectDetection/tabid/176/ProductID/606/List/0/Default.aspx?SortField=ProductName,ProductNameHaven't tried it myself, but looks interesting.
I used a PIR sensor module for the motion detection for a security light and fixed everything in a small transparent plastic box (PIR sensor module also inside the box). It is working fine without any issues. Whenever there is a motion it switches on the light. So I believe it works for your case as well with out any issues from behind the clear glass. Just give a try!!