It will be in, or near to, the first place he started to look for it. This is most commonly the case with lost objects.
What happens is, you automatically go to the most obvious place first, but if you don't do a thorough search there (maybe it's hidden by something that has also been left there since) you start to look in less obvious places. As the panic grows, you start looking in even less and less likely places.
This has happened to me many times. Quite commonly with car keys. I usually leave them on the side near to the kettle. I go to look and they are not there. So then I check the pockets of clothes I wore the previous day. Not there either. So then I start looking in other rooms in the house, not there either. So then I think I might have left them in the garage whilst pumping up my bike tyres. No, not there either. So then I start looking in wardrobes, under the bed, in the fridge, in the attic, etc. I start to remove the skirting boards and lift the wooden floor. Nope, not there either.
Actually, before I go quite to those lengths, I usually look for someone to blame. It starts nicely with...
"I don't suppose you've seen my keys?" to my dearest.
She'll usually reply: "No love, when did you last have them/where did you put them?"
And I'll start getting irritated then and say something like, "If I knew where I'd put them I wouldn't be asking you would I?" And then I'd change my tone a bit and say...
"Are you sure you haven't moved them?" Because she does like moving things does my missus. She's always rearranging the furniture for example. Sometimes I've come home from work and thought I was in the wrong house!
She'll make it worse by saying something like...
"I've told you to put them in the keybox, then you'd always know where they are!"
And I'll snap and say, "They won't fit in the bloody keybox because its full of all your stuff"!
She'll then get up and start looking for them as well. And she'll go to the most obvious place first, the place where I usually leave them. And she'll go...
"they're here!" Like I'm stupid or something.
And I'll go "Where?" like it's just not possible. And she'll be there, stood by the kettle, dangling my keys and laughing, saying..."you didn't look properly - they were just behind the kettle".
That feeling of relief, appreciation and annoyance at myself. It's hard to tolerate. I might have to finish off by saying, "in future, clear that bloody keybox of all your old stuff that you never use, then I can get my keys in there and this wouldn't happen!"
And she'll just look at me and laugh - and start rearranging a bit more furniture!