It's a Kuwaiti Air Force C-17 transporter, the RAF has the same aircraft. Quite a large cargo aircraft that can also carry pax. As for not divulging what it is carrying, does the site always show what's onboard? Even military aircraft have to have a manifest to fly and has to tell every country it flies over, when filing it's flight plan and requesting clearances etc, what is onboard and request dispensation if it's carrying certain types of dangerous goods. However, this could simply be classed as a 'Diplomatic' flight and therefore the people who load and unload it won't even really know what's onboard. Edit, or it could just be empty.
Despite a little knowledge of physics, I still have a problem with understanding why heavy aircraft can actually fly and don't crash to the ground! Is that helpful?
Some data isn't available on ADS-B, which is why there's no destination. You'd have to have access to the flight plan system to see where it's going, but even that doesn't usually detail cargo unless it's dangerous. I see that very aircraft once a week.
I know this is off topic a bit, over the past couple of months I have seen daily RAF ( I think) nimrod (surveillance) air craft flying dissent low over seal sands and the trees side area a number of times taking the same route.
