Okay, I have search the forum and most Bonus chests seem to relate/refer to the chest you get when you have completed 10 summons in Atlantis, but I just got a chest and we are nowhere near Atlantis.
Could it be an unclaimed Atlantis chest somehow? I don’t think I did any substantial pulls during the last AR since i was saving for Avalon. Is there just a random Bonus chest that pops up?
ETA: I did have a look at this post which mostly refers to the items that are found in the chests.