Just done the same thing to mine and it's like Dunccoss said. I used a small flat blade screw driver and dug the sealer out carefully to find the cross head blanking plug that's under there ! Don't know if its me been paranoid but when I take the bars off now I fit a stainless button headed Allen bolt with a smidge of grease and a small rubber gasket/seal in all 4 of the holes before a put the plastic covers back on ! The reason been the covers aren't water tight and the hole you have to dig out makes a perfect puddle for water to sit in probably wouldn't cause any trouble but I just thought it was a crap idea !!!
Hth paul