Game Masters have an ability to turn invisible, where their location is not sent to the client machine. Certain situations arise where people can get in the way or may play a role in an event. The only known way to test for GMs that are invisible is to test out acrobatics and get the message "X number of people" to tell how many people are actually in the area.
This has been successfully done to lure Silvanis out of hiding when RedStorm was attempting to recover the Magnesium Mines.