On page 180 of the 5th Edition Shadowrun Core Rulebook, after the following sentence:
Anyone in the suppressive fire zone or immediately adjacent to it takes a dice pool penalty to all actions equal to the shooter’s hits, unless they are completely unaware of it (a magician using astral projection, for example).
Add the sentences: "Anyone not taking the penalty while in or adjacent to the suppressed area must immediately make a reaction + edge test to see if they’ve been hit. An unaware person walking into the area must make the test using only their edge (such as if a deaf person who couldn’t hear the gun shots nor see the shooter was strolling by). A player may choose not to take the penalty while in the suppressed area, but instead must make a reaction + edge test (number of the shooters hits) or eat flying lead. These penalties apply even when in cover or lying prone, so long as the character is in the suppressed area.