Hard power-off of a Windows machine will ensure that wake-on-lan isn't functional, but there are two possible combinations of bios settings that can lead to machine waking up without power button press:
- Restore on power failure: Always on\Previous state
I am not entirely sure that powering off via holding power button will account for power loss though. If so, machine would power itself back up straigh away within a second or two.
In the scenario when a machine has Always on setting and is powered off, disconnecting and reconnecting the power will make the machine power on.
I haven't seen this particular setting on laptops, as they have battaries, which is an internal UPS in essense, so these conditions only apply when the battery is dead.
- Wake up on Real Time Clock
This one would just schedule power on and wake the machine regardless of other software settings.
Summing up suggestions from comments: any random power-ups with consequent shutdowns are likely a hibernation after sleep. Imagine sleep timer is set to 10 minutes of inactivity, second timer hibernates after 300 minutes.
If you leave your laptop at 22:00 at 22:10 it will fall asleep, at 03:00 it will wake up, dump RAM onto the drive and power off, which might take a few seconds to few minutes, but the machine will spin up the fans and flash all its activity LEDs as it is under considerable load copying data.