So, I recently went on a quick jaunt to London town, and when I got back I reread all 7 of the HP books, because I felt that I had a new appreciation for places like Kings Cross and Tottenham Court Road, etc.
In Book 7, there's a bit toward the end where Harry is entering Dumbledore's office to make use of the Pensieve, and he doesn't know what the password to get in is, but he blurts out "Dumbledore" and it works.
It just occurred to me what a genius password this was on Snape's part, because Deatheaters and Voldy would never guess it of Snape, but Order of the Phoenix might in desperation . . . very clever. Nice little detail.