Was this a pre-paid rate? (I'm guessing not, but just to be sure).

"Direct Bill" is where a stay is charged to someone other than the guest, normally via an invoice. This could be a travel agents, company account, third-party booking site, etc. You mention Expedia - did you ever actually book via them, or just price the room through them?

My guess is that someone went wrong during the BRG and the hotel has billed Hilton (corporate) themselves. I would not presume this wouldn't be picked up at some stage in the future at which point you might be charged, but for now at least it looks like your bill has been paid, and you're good!