Now I'm curious about YukYuk's comments about "a home that spent a seasonal shift closed up (example closed up but not heated in a cold winter, hot summer remaining closed up without air conditioning, etc.)" This is the most likely cause since the neighbors, who had all been in the house, had never seen this in their visits when the previous owners had been alive. It now appears that the house may have been close up for as long as two years...two winters and two summers without heat or AC.
My new question now is: Now that the AC is on (for the Summer) and the heat will be on for the winter, is it likely that the walls could "shrink" back? If the walls return to their "normal state", could the re-skimmed portions become visible or become loose and fall off?
Thanks again for all the wonderful replies!