You'll need to go back to chapter 1 to remind yourself why he hasn't been able to leave. Nono is the "prohibited item" that Alba needed to clear the S-Rank dungeon in order to get permission to "transport."
Honestly I could see it going one of a few ways.
The equivalent of child protective services got wind of a kid being found and would like to chat with said child to make sure things are on the up in up.
Royalty knows about the existence of transmigrators and have contingency plans in place to protect / exploit them.
The exemption item is supposed to be taken from the same dungeon it was aquired in, but this only enforced as such if the Gatherer tries to obtain items that would cause a power imbalance, or the Royalty takes an intrest in what objects the Gatherer is so interested in that they would complete a second S ranked dungeon just to keep.
The entire system is run by a bunch of greedy asshats, and despite cautionary tales, have decided this jumped up Gatherer is being too cheeky and needs a little reminder that he exists at the sufferance of the nation. So he should just quietly hand over the child and a few other bits and bobs as restitution for their magnanimity.
Completely unrelated to anything we've stipulated above and his long absence has delayed a nominated quest long enough the royal guard were sent down to wait and fetch him soon as he showed his face.