Dell Optiplex 3060 Micro auto power-on issue

Hi,

I am experiencing a persistent issue with my Dell Optiplex 3060 Micro (BIOS version 1.30.0): the PC powers itself on and than power off automatically, even after a full shutdown using the command `shutdown /s /f /t 0`. This occurs randomly — sometimes hours after shutdown — even when no keyboard, mouse, or network cable is connected.

I have already performed an extensive investigation to rule out all software and configuration-based causes. Here is a summary of the steps I have taken: 

1. Replaced the CMOS battery (old measured at 2.82V)
2. Disabled all wake-related BIOS settings:
   - Wake on LAN/WAN → Disabled
   - USB Wake Support → Disabled
   - Auto Power On Time → Disabled
   - AC Recovery → Power Off
   - Deep Sleep → Enabled in S4 and S5
   - ASPM → Disabled
   - UEFI Capsule Firmware Update → Disabled
   - SupportAssist System Resolution → Disabled in BIOS
3. Disabled Fast Startup, Hibernate, and Hybrid Sleep in Windows
4. Disabled Intel Management Engine Interface (IMEI) via Device Manager and registry
5. Verified that LMS and Intel ME-related services are not present or running
6. Removed Dell SupportAssist OS Recovery Plugin
7. Verified that no scheduled tasks are allowed to wake the system
8. Verified:
   - `powercfg /lastwake` → shows Wake History Count: 0
   - `powercfg /devicequery wake_armed` → NONE
   - `powercfg /waketimers` → empty
   - `powercfg /requests` → only shows `vncagent.exe` (RealVNC), later disabled for testing
9. Analyzed sleepstudy and energy-report → no wake sources identified
10. Downgraded BIOS from version 1.32.0 to 1.30.0
11. Tested with a known good HP 90W 19.5V adapter → PC still auto-powered on after several hours

Despite all of this, the system continues to power on unexpectedly. The fact that the system powers on from a full shutdown (not sleep or hibernation) indicates this is not a standard wake event.

Does anyone has any idea where to look for the issue?