Black bag

You can see it, but you can't see what it is. A black bag job is one where the bank records show someone being paid, and there may be taxes and paperwork filed, but there's no record of what they were actually being paid for. This could be something as small as a government hiring assassins, or as large as a top secret research project. This term is relatively common in militaristic governments, as well as in Corporations who don't want the auditors to know everything they're up to.