Often wondered why directional adhesive stickers are placed over emergency lights. Surely it would affect the lux? Any thoughts? 

Maybe it is to direct the light to where it is required.

Have you an example picture?

 Its sited on the ceiling. 

Often they are placed on them when they shouldn't be. The exit legend kits are for when a fitting is to be installed as an internally illuminated exit sign -  something separate from the provision of emergency lighting of escape routes, high risk tasks, etc, etc.

Too many untrained installers think because a light fitting includes a legend kit they have to put it on - far too many times I have to go around ripping them off so the correct illumination is provided.

Thanks for the replies. Confirmed my thoughts.