What are some reasons for hand painted dots on chips?
These are put there during assembly and test of the PCB and can mean anything the manufacturer wants to mark.
Often such a dot is put on a microcontroller, when the firmware is successfully programmed into memory. It can also mean, that the device passed all the tests during production.
Whatever the dot means: It is a sign for the manufacturer, that certain steps are done. This makes it easily visible if a board is ready to be processed further, or packaged, or ...