Hmmm... I put my cels in fresh bags right away, 
but given time (weeks, months) in a cel book, 
the part of the bag that touches cel paint 
begins to develop warpage and is no longer 
smooth.  This seems to happen with almost all 
cel bags I've seen.  Look closely.  If you take 
your cels out of the bag, you can almost 
certainly see an outline where it was sitting.  
If you don't want the bag to show in the scan, 
use a fresh bag and just don't use the seal.  No 
adhesive = invisible bag.