I've found the plastic clips actually in the seat rail - they actually slide off the locating tabs in the tracks. This gives a wonderful 'AAARRRRGHH!' feeling on cornering, where the seat runner is neither located top or bottom - it feels like the seat might just tip over!
When you replace them you should clean out the runners of all the debris in there and give them a spray of white grease to help the seat adjust. I suspect the runners will be full of grit and fluff and junk, which is why the clips have come off.