I opened a bugreport a couple of days ago because I assumed it to a GDM cause. Sadly no response, so it also could be a gnome-shell issue.

https://bugzilla.redhat.com/show_bug.cgi?id=1061148

;--- Report

I found a serious security issue which is reproducible everytime here with Fedora 20.

Open gnome-terminal and make it fullscreen (F11)

tar xfjv some big file (say 1gb or something like that).

Under heavy harddrive load go and lock your screen under GNOME.

You get into the "Shield" of GDM (or gnome shell) where you can enter your password, make volume higher or lower and so on.

Now close the lid of your notebook so everything goes into hibernation.

Open your lid of your notebook and press "power" to get the machine out of hibernation again.

Voila you are inside your gnome-session. No password required, you can do whatever you want.

Now imagine you leave your notebook for a coffee break or going to the toilet. Someone could come up and grab your work from your notebook.