You only received a password when you died in Metroid; the password would track how many energy tanks you'd collected, but not your current energy level (because at the time you receive the password, that energy level is zero).
It not only tracked your missile count, it tracked which missile tanks you picked up, as well as which doors you had blown open. There's a lot in there.
You only need a single bit to keep track of whether or not a door was blown open. I'd imagine the missiles worked the same way, and they just computed the total as the number of missile bits set times 5.