When attempting to place a Chemlight under Props, no effect appears to occur when left clicking within the editor and if you view the RPT you will see:
21:24:33 invalid center
21:24:33 Cannot create entity with abstract type placed_chemlight_blue (scope = private?)
21:24:33 Cannot create non-ai vehicle placed_chemlight_blue,
Believe we ran into this in the past via bug 0013908.