I think the slot with the padlock is for one of thosekensington locks
I recently replaced the capacitors in my own 3 year old Samsung monitor, and it was a royal pain to get the back off. I used a plastic knife to finally get it off, but broke one of the tabs in the process. There were 6 total I think, so it still went back together fine in the end. I don't know what sort of tool they use when they repair them at Samsung, but looking at the tabs after I opened it up, I don't see how they could even get to them to depress the little clips to keep them from breaking off.
So all I can tell you is to take the screws out, and slide one of those heavy duty plastic silverware knives around the sides till you can find where the clips are. Then just jam it in there and pull on the back and hope for the best haha. I'm guessing the two screws are at the bottom of the monitor like mine, so start at one of the two tabs closest to the side where the screws were so you have somewhere to pull from.