@Hadski
The kiddo seems to be a bit too busy exploring a town of another world for the first time. It's possible that he wasn't thinking as deeply about what was happening.
(Heck, I didn't even consider that the witch might have detected his power. The manga doesn't explicitly indicate that she did anything like that.)
Based on the town's people's greetings, Charlotte is known to be affiliated with the orphanage. Given the state of the orphanage, Charlotte's request for the witch to teach MC about the materials, and the assumption that the other, older children are off working actual jobs, the witch might have guessed that:
1. MC might have gone to the forest before and retrieved some stuff from there.
2. MC possibly intends to go back there again.
Not having the awareness or deductive abilities of L from Death Note does not necessarily mean someone (especially a kid) is "dumb", especially given that MC's from isekai stories are usually "average" Japanese adolescents or teens.