Michaelangelo had a big crew of painters working for him. The master generally drew the figures and along would come the help or understudys to fill them in. It's conceivable that these helpers added the figures like the figure doing the dump. The same sort of thing is sometimes seen in architecture and is sort of a signature for the workman. I believe they have even found them on the Great Pyramid.
I used to do CADD (Computer Aided Drafting and Design), and on many many CADD files there are humorous (and sometimes raunchy or disgruntled) comments or details, reduced by a factor of thousands or on an obscure layer, or as part of a fill pattern. AS I like my job I contented myself to put my initials as a fill element on an architectural detail, but I've seen some things from clients..... That this was all part of such an ancient tradition I did not know.