Just wondering if evaporation lines fade from HPT's?

After my posts yesturday Im really worried maybe we are reading evap lines?

The line in the test region is soo very faint, its lead us to having conversations about how faint is to faint! :eek:

The line comes up very faint within the test time frame, but gradually gets darker. Like hours later its much easier to see.

So Im hoping the evap lines disappear, thus making it a true +.

Does anyone know? Or do evap lines get darker over time too?