What do art historians mean by the use of "hidden symbols" in the mérode altarpiece (fig. 19-10, fig. 19-11)?

The revering of Hidden symbols in mérode altarpiece by art historians mean those simple objects might have more to it than anticipated. So it simple means that the normal objects which also have some symbolic meaning and this kind of thinking enables them to widen their research.

Normally it is at the top left, above where the music starts.