Theres no other fish in your tank? Could it have been picked on and passed away? Depending on when it died a decent clean up crew could have eaten it.
Did you check your filter and around your tank? Fish can get into the smallest spot sometimes you really can't imagine. LOL
As a side note, and not to make it worse, but a tang shouldn't be in a 65g tank. theres a couple tangs that maybe can get away in a 75g but a 6 foot tank is needed for almost all the rest.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.