In LLPSI, one can read:
Medus, qui Graecus est, in patriam suam redire vult.
I would have expected
Medus, qui Graecus est, ad patriam suam redire vult.
Indeed Cicero uses "redire ad se atque ad mores suos" and Caesar "ad neminem unum summa imperii redit".
How can one explain the use of "in" here?
There's a difference between the two. Using ad properly would indicate that Medus wished to go up to but not necessarily into his native country. This is why it works with people: you go to people, but not into them.
When you're talking about entering, though, in is more clear about movement into the place, such as in Livy 11.1:
et tum quidem ab nocturno iuuenali ludo in castra redeunt.
and then they indeed returned from their nocturnal juvenile game to [=into] the camps.
You can see other examples by checking PHI or Lewis and Short's dictionary entry, which has a great example of the two in Curtius Rufus (5.5.20):
se rediturum ad penates et in patriam
...to the household gods and into the fatherland.
