I found this one at the bottom of my Lenovo X61s laptop. Lenovo has a long tradition to label all the screws what they are for. So one doesn't end up unscrewing the whole thing just to take out the keyboard for example. On my Lenovo there's also a keyboard sign with a droplet on the top with no screw besides.
I thought that this might be for spills and after some research I found that with T60 series Lenovo redesigned the keyboard so that if some liquid is spilled over it is caught by rims under the keys and drained out through the canals at the bottom of the keyboard. I wouldn't dare to try it though!