Most likely the plug was stripped before you got it. They don't just fall out. When I said they sometimes do, I meant as the result of improper installation; too much torque, too little torque, wrong plug, etc.
I hear ya. I bought this truck used in 09 and haven't needed to do anything to it, so whatever has happened was begun long ago. I've put 30k miles on it.
And an update is in order: The dealer says: yup, it's stripped. The force of the plug expulsion not only shredded the COP boot, but also cracked the coil housing, too. So far, the estimated cost is roughly $185 and an overnight stay.
Service writer originally told me $100 to fix, mech had $200 on the clock - writer bumped that back to the original $100. Insert kit (he forgot to mention) $45, COP $85.
Thank you all for your helpful comments. I surely hope that the old owner or *their mech* didn't cross-thread any more sparkplugs.
Edit: I forgot to mention that when this all occurred, to me (in the driver's seat) it sounded much like a tire blowout, and then an open exhaust header. And in addition I discovered that my air conditioning quit at the same time. The service writer would tell me that this had more to do with an engine misfire fault than with an air conditioning issue. I later discovered he was correct.
The dealer should have installed a Lock 'n Stitch insert, not a helicoil. Don't confuse the two. It's not necessary to remove heads to install inserts on the Ford modular engines, the service can be performed in-vehicle. Several cylinders are easy, several are tough but doable.