Often a case for pareidolia. The oval shape on the reverse of one of the uprights Stone 52 in Trilithon One is many things to many people.
Here are close ups without skins of colour, mould, lichen, most to get in the way.
Trilithon Four of Stonehenge
Stones 57, 58 and lintel 158. Outer is left lumpier (right), inner is polished flat (left).