It is possible that the thief redeemed the XMR that the coins were funded with and threw the coins in the trash (this would result in less evidence to lead back to the thief).
Miffman was provided the digital certificates that prove the coins were funded.
If indeed they were redeemed, their key images would be spent and searchable on the XMR blockchain.
As far as finding a trail back to the thief, I would imagine that would be a difficult task to accomplish given how opaque monero is. This is the first known theft of any of my coins that were shipped out, so perhaps I am wrong.
I just checked the key image of each coin, and they're all unspent. I'm not sure whether the thief even knows how to redeem the coins, since the instruction paper was left in the envelope.
Thanks for letting us know.
I'm of the same mindset wondering if the thief knows how to redeem them or not.