something's feels totally off
That'll be your inability to cut the apron strings. No wonder he's planned it without telling you and has kept her at arm's length from you, if this is how you react to things. He's 21, a grown ass adult and he's moved out. You haven't had a row, he hasn't said he's going no contact with you, so what exactly is the problem?
We normally share everything
I had a parent like that. It wasn't a good thing. It was a not being allowed any boundaries thing, a control thing, a lack of privacy thing. Mine doesn't understand why they don't know where I live either. They also didn't understand when I grew up finally, developed a backbone and stopped sharing everything. They became very angry, then upset, then confused. They'd swear blind we had a good relationship and have no idea why I "suddenly changed". It wasn't sudden, it was a decade long battle for autonomy that was only achieved by going low contact and keeping personal information private.
the only reason why I did a drive in the general area is to see if I could find where he was living for my own peace of mind
OMG WTF have I just read?! That's stalker behaviour, seriously. Your desire for peace of mind doesn't trump his desire for privacy. Respect his decision to not tell you where he's living and don't try to find him or demand to know. You don't have any right to know, it's his decision if he wants to tell you or not. He's said he'll call you, so wait for his call.