Had a look at work. It's not the same model and I can't see anything obvious.
However I've changed motherboards on laptops, which means taking all the screws out. Basically till your down to the components on one side and the shell on the other.
A few places I've missed before are under the keyboard, at the side (next to network ports or CD drive) or right inside a small deep hole on the bottom (usually in the center).
Lets be honest though, it's either a missed screw or a catch mechanism. I know, it could be anywhere and it really annoying.
If I come up with anything else I post it for you.