"secret"

I've always gotten more of a "everyone knows who they are but not what they do" vibe from shield

The next time someone tries to convince you of the Illuminati, show them this. Secret societies stay secret by NOT advertising their presence.
.
If there really is a globe controlling cabal, you will never know about them, their existence or how they operate. They would be perfectly happy staying secret, quiet and anonymous. They would also be very happy with a supposed Iluminati, et. al. because it draws conspiracy theorist nutjob attention away from what is really happening and on a fruitless exercise.

but nobody knows what the logo actually means so..

It's so overt, it's covert.

http://smg.photobucket.com/user/Unicron55/media/RPG/BTech/Houses/freeworldsleague.jpg.html
Every time I see the SHIELD symbol I think of that.

Skye/Daisy mentioned it in season 3