Windows 10 keeps waking up from sleep on its own – no idea what's causing it

You already disabled device wake sources and timers, but powercfg /lastwake is returning “Unknown” or a device. That is a clue, not a diagnosis: it reports last wake and may not name a scheduled task or firmware event. After the next unexpected wake, run powercfg /lastwake, powercfg /waketimers, and powercfg /devicequery wake_armed in an elevated terminal, then save the outputs with date and time. Generate powercfg /systemsleepdiagnostics too; it provides a history of sleep transitions. Check Task Scheduler for tasks configured to wake the computer, especially update, backup, or vendor maintenance tasks. In Device Manager, compare the devices listed by wake_armed with the devices you disabled; docks, Bluetooth adapters, and network controllers can re-enable wake after updates. Update BIOS, chipset, and network drivers from the PC maker. If reports still show no cause, test one night with Ethernet and external USB devices disconnected, then reintroduce them one at a time.