Découvrir le pot aux roses

This charming expression involves roses, presumably sitting in a planter - pot - which must have been hidden because at some point someone discovers it - découvrir.  Découvrir also means to take the lid off, but since flower pots don't normally have lids, we will stick to the scenario where the rose container is a bit out of sight and it takes some determination to find it.  Literally speaking, j'ai découvert le pot aux roses! translates into English as I found the pot of roses!  What you really mean, however, is that you found out a well kept secret, you uncovered a plot.

Note that le pot aux roses sounds like "le potorose" because of the (not so dangerous) liaison between the otherwise silent t of pot, and the vowel sound in aux.  If you didn't know any better, you might understand "le poteau rose" - the pink post - which might be even harder to find than the proverbial rose container.

Origin

Most sources date this expression to the 13th century, although linguists have suggested various theories.

The rose as a symbol for "secret" dates back to Greek mythology.  It started with Aphrodite giving her son Eros a rose.  Eros then gifted the rose to Harpocrates, the God of silence, so that the latter would silence the amorous adventures of Aphrodite in exchange for the gift.  This is how the rose became a symbol of secrecy and silence.  Romans decorated their banquet rooms and party halls with roses, to remind attendees that everything that happened there was to remain private.  The symbolic meaning of the rose lasted throughout the Middle Ages.  In the rooms where bishops of the Catholic Church held their assemblies, a rose was hung from the ceiling to indicate that the debates engaged “under the rose” were to remain confidential.  Hence the Latin expression sub rosa which literally means “under the rose”, that is to say “under the seal of silence.”

Now, what's with the pot?

In the Middle Ages, le pot aux roses designated the small box where wealthy women stored the rose water they used as skin care and fragrance.  The jar containing rose water had to be kept covered to prevent the water from evaporating.  The verb discovering in this expression could therefore have been intended both literally and figuratively.  When the lid was lifted, the pot revealed the secret of a woman's beauty - or the artifice explaining her beautiful complexion.

Some sources report that the rose water box was also used as a secret hide for love letters and other private mail which a woman would want to keep out of husband's sight.  If her husband found out, that would have qualified as a discovery indeed.

Examples

Laure a découvert le pot aux roses quand elle a lu les messages entre son mari et sa maîtresse.
Laure found out what was going on when she discovered the messages between her husband and his mistress.

Ses amis ont comploté dans son dos pendant des mois mais il a finalement découvert le pot aux roses.
His friends plotted behind his back for months but he finally discovered their machinations.