How about to relieve some of your Stress I give you my own input on this as Rawr himself.
Yes, it's a largely known exploit in which I've told Admins of TF2SS before, yet I don't remember which one. As it was long ago.
It's a recording exploit, and no I do not personally use it myself as using it will take up harddrive space that I need for other things.
The exploit itself is this as following:
In console put in the following command
bind key "record 123; stop"
This will force your game to record a demo for a split second and then stop it. And during the second it renders/unrenders the demo everything in game is rendered. Players, particles, and yes, invisible spies.
And during the time this was shot. Neither Syn or I actually used this glitch.
And as to my knowledge you won't be able to stop it in your own servers unless you turned off the recordings of demos in your servers.
Hoped this helped.