I looked at my newish Limbo IATA multiwheel and there is a very subtle line similar to what you posted at the top when it's upright on its wheel. It is very subtle and I did not notice it with the naked eye until you brought it up. Also I had to get my face within a foot of the case. I would think it is an imperfection in the polycarbonate. (It's possible it was caused during my recent travels.) It's the only line that I can find. I will have to look at my larger 30" Limbo to see if there's anything similar. It does not bother me because the case is structurally sound and I have some peace of mind with the 5 years warranty.