I don’t know about “hidden” symbols specifically, but there are many traditional symbols for the Mother of God. These include a heart pierced by a sword or swords (echoing the words of Simeon in Luke 2: 34-35) and the mirror, a symbol of her sinlessness. Mary has sometimes been represented by the image of an enclosed garden, symbolizing her purity (this comes from the description of the bride as “an enclosed garden, a fountain sealed” in Song of Songs 4:12). Speaking of gardens, throughout the centuries many flowers, plants, and herbs have come to be symbolic of Mary’s different attributes. It would take too long to list them all here, but they include the lily of the valley (said to represent Mary’s tears) and the violet (symbol of her modesty). From the twelfth to fifteenth centuries in particular, many artists painted images of Mary in garden settings, surrounded by or holding the flowers that represent her.
In addition to symbols, there are also Marian monograms. These are usually abbreviations of her name or a combination of letters found in her different titles, such as MR (standing for Maria Regina, or Queen). Another common monogram is the letter M with a cross in the center, an image which recalls Mary as Our Lady of Sorrows.