When Deidara activated C0, Tobi and Sasuke were in range.
The range was so huge that everyone including Zetsu thought Sasuke was dead.
But when Itachi was looking up at the rain, he automatically knew that Sasuke was alive after the storm has passed.
I think that what happened was that Tobi who survived the blast informed Itachi using the storm that Sasuke actually lives using some kind of storm technique.
Plausible Theory?
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