Red background fllash at startup. iMac 2015

iMac (Retina 4K, 21.5-inch, Late 2015) new in 2016. 3.1 GHz Intel Core i5, 8 GB 1867 MHz DDR3

OS 10.11.6

At startup a thin red lightening like flash appears briefly in the background, then disappears. Then system boots up.

Has anyone else had this occur, know what causes it and/or how to fix it? I'd appreciate any information.