its the battle in Starfield, everyone exept you went somewhere, Robin, Steve, Erica and Dustin went to Dustins super radio tower. Joyce, Hopper and Murray went down to the russian base and the rest went as a bait for the mind flayer to folow. You stayed in starcourt so that you woukd be safe since mindflayer was coming after you. Because you are 010. You still are on the walkie talkie so that if someting happened you or the others could reach eachother, but you didnt know that Billy was there with you the entire time