The Mysterious Loss quest is one of the Meteoric Wave missions for the Star of Destiny in Genhin Impact’s Unreconciled Stars event. You will need to have the 7 previous Meteoric Wave missions done before you can access it. You will also need to have completed the What the Skies Conceal, the Waters Reveal quest.
When you do the mission, you will need to head to the Catherdral in Mondstadt and speak with Viktor. He will be just inside the door on the left. Speak to him and he will ask for your help recovering a box that he dropped when he was attacked by some Electro Slimes.
Agree to help him, then open up your map and head for the waypoint that is marked near the Thousand Winds Temple. When you get there, use your Elemental Sight to find the first trace of the box. You should be able to see a purple trace on the hill just behind where you spawn in.
Interact with it, then turn and move up the hill, along the path to the left. You will see a group of Electro Slimes up there, and these are a bit of a giveaway. Take them all out, the use your Elemental Sight again. You will see another purple energy marker. Interact with that to find Viktor’s box. You can see the exact location marked on the map above. You will now be able to return to the Cathedral and give him back his box, wrapping up the mission. You will get to ask him what he knows about the Fatui and the meteorites after you return the box to him.