I'm sure Grant Thornton would be happy to do a full audit of Circle's assets IFF Circle was willing to hand over all of their banking/brokerage statements and allow them to contact those institutions. Verifying the quantity of cash and Treasuries sitting in an account, the length of time they have been there, and the counterparties to their largest transactions is trivial and doesn't compromise any trade secrets which is why it's such a huge red flag for Circle (and Tether) that they refuse to do this while continuing to tease that they will do it at some point in the indeterminate future.