After the kill the sign does not appear unless you refresh the screen. Only then does the icon of the spider disapear.Make sure you read the sign after you kill that beast.
This is similar to IG elsewhere where after a kill the Icon remains but IG usually items appear afterwards. I have take as how the game works and do not know if it is fixable but the sign can easily be missed in the training center.
This was all done using IE8 as my web browser.