Any thing that is placed in front of the fresnal is going to decrease the detection somewhat. I have tried mesh camo and other have tried different things that I have read in the postings but I don't recall anyone saying that their method didn't affect the detection distance. Even the colored fresnal lens I have read where the distance was shorten a little using them.
Can't remember for sure about the marker (sharpie pen) did or not. Use it on the inside if you try it. I'm betting it decrease the distance also.
I would hesitate on putting anything in front of fresnel lense. I put my cams on trails only and sometimes get background critters out to 70 ft. so I started putting them up higher on trees and shiming them to point more down. This seems to help.
I was looking for something to put over it that would weatherproof it. In other words set the fresnal lens within the case about 1/2 inch from the front and then put something over the front of it so it is fully enclosed.
Reason being, I am using the Radio Shack 426 sensor and it says to be careful about weather on the fresnal lens.
I tried one cam using some of the "blank" lens from fresnal tech, it did seem to shorten the distance.
If the fresnal lens is about the same as the 425 pir that preceded them they will be ok. The 425 seals ok. As long as there are no holes in the lens (I haven't used a 426) they will protect the cam, unless something bites a hole in it.
They were probably talking about the sensor as a whole, I wouldn't try using the sensor housing by sticking it out the front unless you plug all the holes in the housing with some type of glue or sealant.