The air stagnates below the wiper blades and moisture run-off tends to collect there. Once the air temperature is below freezing, the collected moisture freezes, unsurprisingly. It will then remain there until it melts; either in direct sunlight or more often in the descent (after n hours) when the air temperature rises. Ice often forms under the wiper blade on a CAVOK day due to condensation on the nose running back.
Practically, ice below the wiper blades on a 737 is not a useful indication of airframe icing.
Ice on the wiper bolt is better and severe icing is often indicated by icing on the windscreen on the middle and outer sections of the front panes. Next time you see it on the bolt or window, check the leading edges and then apply wing anti-ice and watch the ice fly off.
Have a look here
Ice & Rain with thanks to Chris Brady.