Even if the lid somehow managed to go perfectly back in place.. apparently there is a square of sod that was over the lid as well, as seen and reported on by a news reporter.. if that piece of sod had been somewhere other than covering the septic-lid when police first arrived, then LE would have noticed right away at first inspection of the yard.. and that would have been a large clue that something could have happened, and I'm sure they would have made a decision to pump it right away. Somehow, LE, although it is stated they checked the septic a few times, decided it did not warrant pumping until day 5. Although a lid could potentially flip back into place after devouring a human by accident, the sod has no such tricks up its sleeve.