I've had a thought on the back of my mind recently. It's a sign, like an access, restrict, lock, or preview sign, that had [SENSE] on the top row. Basically, this sign will tell you who has gone over a certain block. For example, if someone went somewhere they weren't supposed to, the res owner/admins would know. The player is recorded in a book and quill that must be in a chest underneath. The sign can be used without a form of redstone, and nothing will show up in chat if someone tramples over it.