After watching the press conference in full and researching this case essentially from the beginning as it's in my home state ( and I've felt it was off from the very beginning), I have a thought - all just speculation of course. Could it be possible that, Jess and JM were out tending to the sheep and have both been under the impression all this time that Gus had wander off (or been totally in the dark as to what happened in any scenario), but of late perhaps they have (either both or one of the party) been suspicious of SM and given the police information as it has surfaced? Perhaps they too were in the dark until most recently of anything that had happened that night, believing only what was initially told to them via SM?
For the police to outright say that Jess isn't involved, I would assume that they are satisfied she was not present during the time of Gus's disappearance. I would also assume that Jess would have no motive to lie about being with JM during this time also. Perhaps just recently SM has confided in JM some information, or perhaps either party has become suspicious themselves of SM, maybe in their behaviours or even found themselves something of evidence they have provided to SAPOL which has lead to a search warrant being undertaken. Is it possible this is why JM is also needing legal representation as they are providing information about the other party?
The AI imaging really was quite remarkable, the search radius was extensive. I feel very proud of SAPOL for undertaking this case with the extreme diligence & thoroughness they have, they weren't going to let this rest without result. This is of course all just my thoughts, and speculations, only based on possible scenarios I think could have happened.