Wiktionary's etymology of the term sub rosa reads:
The rose's connotation for secrecy dates back to Greek mythology. Aphrodite gave a rose to her son Eros, the god of love; he, in turn, gave it to Harpocrates, the god of silence, to ensure that his mother's indiscretions (or those of the gods in general, in other accounts) were kept under wraps. In the Middle Ages a rose suspended from the ceiling of a council chamber pledged all present – those under the rose, that is – to secrecy.
Does anybody know which part of the world the above-mentioned council chamber used to be in? I'm also interested in any other related information.