Not to double post, but today's Daily Deal at Parallax is their X-Band Motion Decector;
http://www.parallax.com/Store/Sensors/ObjectDetection/tabid/176/ProductID/606/List/0/Default.aspx?SortField=ProductName,ProductName
In the write up is says it can see through walls... never used this sensor so I can't attest to that, but if your lexan isn't clear, then this should work still.