He could be being groomed - the adult will go through various stages, getting child's trust, getting child to think they can only trust the other adult, creating problems between child and parent, talking to child at length to find out personal stuff etc.
How your DC has reacted would freak me out a bit too, though based only on my DC, as at 10 if someone had put things in my DC's bag my DC would want to get to the bottom of it and find out who and why, they would be more bothered by it than you are. I would find it very worrying if DC clammed up.
So it might be nothing, but I think you should trust your feelings on this. Have you ever spoken to your DC about things like grooming and the pants rule and things like that? There are some good books for kids about it, for parents to read with kids. If it is grooming, it could be anyone - anyone you know, sadly, or a stranger depending on whether this is possible for your DC. Telling your DC in a child appropriate way about grooming and about how if anything at all happens similar with your DC your DC does not need to worry and can tell you everything, at any time, no matter what your DC has already done, and you can deal with it, and it will be okay, might be a good start.
Going back to grooming, there is info on the internet but things I have read about before involve a process, so after getting a child's trust and putting a wedge between child and parent, over a period of time, all very innocent and reassuring at first, they then do things like showing pictures of children being abused to normalise it, progressing to abuse but making the child feel it is their responsibility, that the adult was only doing what the child wanted, gaslighting, manipulating both by promises and/or threats to get child to say nothing for as long as possible.
Hope it isn't that, obviously. But a pretty damn weird note, I agree, in the context of your DC not wanting to talk about it.