Unhook a battery cable and put an ammeter in the line with the key OFF. If it shows power going through it, start pulling fuses 1 at a time until it goes away.
It's possible that it's designed to keep something running full time. One thing that can draw power when it's off is a security system. Or maybe an underhood light that isn't shutting off.
This! Don’t use a cheap amp meter.
Also the fuse trays are known to go bad and cause parasitic drain. Had that on an 08 Suburban. New fuse tray, no more problems.
