Your statements contradict each other…
First … ” They key is now dead, battery has good volts ”
Second … ” The battery is also completely drained (3.5V), leading me to believe there is some draw on it. ”
Tough to come up with a diagnostic plan when you offer two contradicting explanations .
If there is a current draw you need to find which circuit is causing it.
Checking all the fuses for current should narrow it down and you’ll need a wiring diagram for your specific vehicle.