How does a 10mm socket end up riding around inside a tyre tread?

A socket wedged in the tread and staying put is totally normal, no soft rubber or worn tread involved. The tread grooves on a car tyre are actually pretty deep and the rubber is stiff, so anything that gets in there at the right angle just gets pinched in place. Every rotation squishes the tyre flat against the road and the walls of the groove close around whatever's stuck in it. That's exactly how nails stay in tyres for weeks without leaking air. Your socket whould've only dropped out if it hit somthing that pried the groove open, like a kerb or a pothole edge at the wrong moment. I had something similar years ago with a bolt from a fence repair job. Sat in the centre rib of my ute's front tyre for months and only came out when I was washing the wheels with a hose. Same deal, no puncture, and the tyre went another couple of years no worries.

Since yours came out clean with no visible cuts, I wouldn't stress about replacing anything. What I'd do is give the tyre a proper look over anyway, not just where the socket was. Run your thumb around the inside of the grooves and feel for any cuts or bulges, and check the pressure hasn't been slowly dropping. If it holds pressure and the rubber looks fine, you're done.

The part I find funny is how it found the tyre at all. The odds of the groove lining up with a socket sitting in the driveway are genuinely slim, and then it stays put for however many kays. Sounds like that 10mm just wanted to come along for the ride before turning back up.

Lost a 10mm is basicly a rite of passage. I keep a bag of cheap ones now and just accept they vanish into the void. Mine turned up once inside a wheelbarrow tyre of all things, must have bounced in off the shed floor. Went months before I noticed it rattling around in there. Only thing I'd add is jack the wheel up and spin it by hand before you call it done. If anything rattles loose out of the other grooves you'll hear it, and better it drops in the shed than on the road at highway speed.

Same experiance here, that second reply nails it about things just staying put. Mine went missing for the better part of a season and turned up inside the door track of the ute, must have flicked in there when I chucked it on the tailgate. Only found it because the window started jamming halfway down.

And yeah, the driveway pickup thing checks out too. Lost a spanner once to the front lawn and the mower found it for me two months later, threw it clean over the fence. Small metal bits go where they want, no point fighting it.

This is doing my head in a bit because I can't picture how a chunky metal thing even fits into those grooves in the first place. I figured the gaps were only a fingernail deep. My story is I lost a little spanner last summer and never found it, so now I'm wondering if it's living in one of my tyres too. Does the rubber sort of close over stuff like that so you can't see it from the outside? Also does a socket sitting in there mess with the balance of the wheel at all, like would you feel a wobble when driving?