I read about a woman whose toddler swiped her car keys. She put a different key in the same place she’d left the old one, spied on him, and sure enough he took the new key to the hiding place of the old one!
It’s not exactly your situation, but I hope it helps.
I have a feeling that he put it in the camper in the yard. Last night he went in there looking for it twice, but it was dark. I can’t go in there- claustrophobia, dark, cramped… can’t. So he will go in there when he gets home from school, before the sun sets, and I will cross my fingers. If it’s not in there, I will see if I can unscrew the hasp. If that doesn’t work, there’s a place nearby that rents tools and I will get a bolt-cutter. I guess I will still have to cut it off even if I can unscrew the hasp, because it still has to be replaced and locked up.
It’s not that disgusting, you put the poop down somewhere, like on concrete, then empty a bucket of water on it - from as far back as you can stand - and then you can inspect for swallowed objects. You may get some residual splash back, so it’s an idea to wear a full haz-mat suit, or wellington boots, whichever you have to hand.
In some small ways I’ve led an adventurous life.
Alternatively you could say that Santa is looking for his key, and ask if son knows where it is…?
Update- my procrastination has finally paid off. Today I got around to asking my neighbor if he has any bolt cutters (much better than buying or renting some, no?) and he asked me what I needed to cut. I explained the story to him, and he asked me if it was a blue key, and when I said why yes, yes it is a blue key, he said that he saw it in my yard this morning, and walked right over to it and picked it up. It was very fortuitous, as he was only in my yard this morning because our water went out (we share a well), so any other day he wouldn’t have known it was right there.