Working as designed
It is little known, that the VNCviewer can listen at port 5400 for flash purposes. To verify it, start a VNCviewer in listen mode. Then telnet to 127.0.0.1 5400. Whatever you type will appear flashing fullscreen on the viewer PC.
This is inherited from the very first implementation of VNC at Olivetti Research Lab. They had implemented a roaming freeseating, where everyone carried a radio badge of a kind. Whenever they approached a terminal, their current desktop would appear on it (through a winvnc -connect ip adr.). If you were in a room with many PC's you needed a way to identify the PC with your desktop. Hence the "Flasher" listening port at 5400.
Could be used for teasing