What
@Igofar said in post #2 is spot on.
It gets a bit awkward because you cannot lift the handle up easily because your hand is still holding the key in the lock, or your key is on a bunch which get tangled with the handle ! The other part of the mechanism that holds the lid down is largely irrelevant - except if it is open, it gets in the way !
But the really important bit is to know how to put it back on again.
Lift the case onto the rack, but position it further forward than it needs to be, and then slide it backwards. That locates a couple of tabs which keep the front end down. Then that handle has to be lifted so that the rear end can drop into place.
Finally, the handle is pushed down, which moves a sliding drawbar across which locks down the rear end.
Until you get used to opening the lid and closing it, it can be very easy to lock the lid, but leave the handle open a bit. Mine has a bright orange sticker on the upper surface of the handled which says 'UNLOCKED', and if you can see that, then the box isn't locked.
It is also possible for the handle and the lid catch to be in the proper position, but for the box to be not seated fully on the rack, so the rear end is not secured.
But get into the habit - every time you lock the top box to set off - just check by trying to lift if up at the rear end - just to make sure that you haven't accidentally left it insecure.