Not sure about the 737, but the Airbus (330, 340, 320) have similar markings (very small blue colored "+" marks).
These marks are used by maintenance and aircraft recovery personnel to check for damage, misalignment, etc, following a hard landing, severe turbulence encounter, off-airport landing, etc.
They take precise measurements between the various marks "+", and compare to acceptable values. From there, they can tell if the aircraft is bent or not.
Again, not sure about the 737, but......