I have the exact same problem. I remember finding this exo randomly during the quest passing through there to the big exo, and activate it. I thought it counted but now it seems I am missing it. UPDATE I started the new expansion with an other character and when visiting a lost sector, I saw that there was available for scanning a dead exo. I scanned it and it credited the one missing. I don't know if it just randomly happen to be available for scanning again or it was due to the different character but it is worth trying visiting every exo location with an other caracter.