-snip-
Is the Android phone still intact? Assuming it's not virus, bug or hacker fault, one possibility is Mycelium send remaining Bitcoin to change address (which generated by Mycelium).
It doesn't look like a change address at all because it's the only output of that "
49.5BTC" transaction.
It's more like a consolidation or sweep transaction.
I'm sharing my friend's story, and we really appreciate your help to solve this.
There's a possibility that he swept it (
sweep) instead of "
import" because the transaction had sent all of the address' available coins at that time to that address.
If that's the case, it should be in the wallet where he used "
Sweep".
One scenario that it might happen is: when he's trying to import it to another wallet like Electrum, but since import is disabled for a standard wallet, he used sweep instead which sent all of his balance to one of that wallet's address.
Wherever it is now, it looks like it's not in his paper wallet nor Mycelium anymore because he said he didn't recognize the address and he can't find the balance in Mycelium.
With that, try to remember any other wallets that he might have used specially those with "
sweep" function, then look for any related backup.