Why does the parasitic draw reading start high and then drop?
Short answer: Modules stay awake for a while after shutdown and then go to sleep in stages, so the draw falls over time; read it only after the vehicle is fully asleep.
When you shut off the vehicle and close the doors, many modules keep running. The body module, infotainment, telematics and network gateway may stay awake for several minutes, sometimes longer, waiting for a door open, key fob signal or remote command. Current might start at an amp or more and step down to tens of milliamps.
That is why a quick reading right after shutdown can make a healthy car look like it has a drain. It is also why opening a door, unplugging a fuse or connecting a meter in series can reset the timers and wake everything up again.
Plan your test. Set up the clamp or meter before the vehicle goes to sleep, latch the doors and hood switch, keep the key fob away, and record the current over time if your meter or scope allows. If the draw never settles, note which steps it skipped; that points to the module or network that is not sleeping.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.