The Nimsoft Robot (nimbus.exe) was running in the foreground - command (cmd) window displays on vsphere/VM console session/desktop.
The Nimsoft Robot Watcher service had "Allow service to interact with the desktop" selected for a single robot. Once it was deselected, the issue was resolved and the cmd window no longer opened up on the desktop. This option should NOT be selected, and it is NOT selected by default. Deselect it and Apply, then restart the robot.