Jesse Billson and acabanillas00, you've run into Apple's new tamper-evident screw stickers. Before you proceed, please note that removing these screws will leave behind clear evidence to Apple that you've been inside the computer, leaving you no wiggle room on your warranty. To remove the screws, first press down with the T10 screwdriver around the centers of the circles, where you would expect the holes to be, and you'll tear right through the stickers (you may have to blindly rotate the screwdriver a bit to get it to line up correctly, since the stickers conceal the holes). Then, just unscrew as normal.
A couple more notes: new Mac Pro 2013's started shipping with these CPU screw stickers some time in 2015, and if you take an older Mac Pro 2013 in for repair, Apple will quietly add the stickers if, during the repair process, they end up stripping the computer down to the CPU bracket.
I haven't found a satisfactory method for preserving the stickers, nor a replacement, but I imagine it could be done...