They were looking for classified documents Trump absconded with.
It's document recovery of classified material. Not a broad investigation.
Apparently they knew where to look, so they had prior information from an insider about where he was hiding things.
Now the question is, what was he hiding? Was it evidence of his guilt? Saleable secrets? Or just love letters from dictators?
He had returned some boxes of information previously. He must have clung on to others. If he hid sensitive information, then he may be banned from holding any government office in future.