I'd suggest pulling the coil & checking the base for cracking.
"missing" under these circumstances is the first sign of a coil going bad. It's an unusual issue for cars in general, but we seem to see it fairly regularly on the Zetec's.
Search here a bit & you'll find some similar threads, "coil cracking" might find 'em.
"Rubber Side Down"
Focus Takeover #003 / Tarbo Fan #003