Just a couple questions. Were you in a fireteam and did anyone open the chest before you? the bonus/secret chests will despawn after about 5 minutes.. maybe less. Another question is if you started fresh or did you load into a checkpoint? I didnt think checkpoint would prevent the chest from spawning, but in the past there has been some strange behavior between the loading areas between 2nd encounter (cube) and pink hallways between the 3rd area ( rainbow road with the chest ) Just some theories why it may have happened