When Noise Looks Like Signal

I was pretty excited when I first measured number distribution of galaxies for my filament candidate stack. In the figure titled Signal to the right white/red represent more galaxies continuing to purple/black representing fewer galaxies. Right where it is expected there were more galaxies (along the filaments).  Now compare this this with the figure below titled Null.  These two figures look very similar.  This is a problem because in the bottom figure I have stacked clusters pairs that are so far separated in redshift space that they should definitely not have a filament between them.

First things first; understand why even in the null case there is what appears to be a filament signal.  I have two primary suspects:

1. Some of the postage stamp regions extend beyond the survey area. Thus when adding all the postage stamps  together there will be fewer galaxies at the larger radii from the filament axis simply because not all postage stamps had  observed galaxies at these larger radii.
2. This is just due to the expected signal from two over lapping clusters. 

Second things second. Correct for this systematic noise. Theoretically I know how to correct either of the above effects.  Now practically may be another story.