OK.......the one you have may have a design flaw. Look at mine, with the single bar treadle......and look at yours with the large, wide pan treadle.
The builder of mine says on the wide pan type treadles (like yours), if enough rats collect on it, they can weigh enough to open it up. On mine, the single treadle won't hold that many rats, and they are spaced some distance from the opening so even if they did collect out there, they can't reach the pan. If they jump off to get to the feed, the cover closes.
In addition to the weight, the one I have also has a spring to hold the cover lid tightly closed so they can't push it open either. Can they push the cover lid open on yours? If so, perhaps that is what your rat did. Pushed it open, then hopped in and was trapped?
The builder of mine says on the wide pan type treadles (like yours), if enough rats collect on it, they can weigh enough to open it up. On mine, the single treadle won't hold that many rats, and they are spaced some distance from the opening so even if they did collect out there, they can't reach the pan. If they jump off to get to the feed, the cover closes.
In addition to the weight, the one I have also has a spring to hold the cover lid tightly closed so they can't push it open either. Can they push the cover lid open on yours? If so, perhaps that is what your rat did. Pushed it open, then hopped in and was trapped?