If they didn't spot the rogue, they don't know about it.
Of course, they do know about the magic aura still, and should be reminded, if the players have forgotten, the characters surely wouldn't have, as this was just a moment before, right?
In the summary I would only mention the PCs who actively did something everyone knows, like A, B and C (D being the rogue) where battling the monster, blah, blah. Don't tell them anything about what the rogue did, don't tell them anything the rogue did not, either.
If - and only if - they specifically ask about the rogue, just tell them, they were busy fighting and didn't watch him. Otherwise, simply do not mention the rogue, and focus your summery on the stuff that everyone clearly noticed.
Conclusions about why the item is suddenly gone are left to the players, of course. I would suspect them to find out pretty easily what happened, even without any specific clues, if they aren't completely dumb.
Could very well happen, that the (still running) detect magic catches the same aura again... carried by the rogue, if he concentrates into that direction long enough!
Bye
Thanee