Secrets

I observed this note on the door in The Fortress Of Lappeenranta. It says

"Bad "secrets" float around and eventually come to light—they never stay hidden. But true secrets, the ones that are strong and meant to be kept under lock and key, are held tightly. You never hear about them, and they don’t drift in the winds. They always remain hidden, and their existence is never mentioned."

What's behind the door?