My 2015 E250 Coupe emergency brake assist activated and brought me to an abrupt stop for no apparent reason. An image of the car came up with a large triangle of red from the left rear of the car but here were no cars close enough to my car in the vicinity of my car to explain why the emergency system activiated. . A few minutes later a car stopped abruptly in front of me (it was peak hour traffic) and I jammed on the brakes but hit the car. The emergency brake assist system did not activate. Can anyone explain any of that to me, especially why the emergency brake system did not activate?