I have a computer at home which sometimes automatically powers on during the night. Windows event log was of  no help. My main suspect was Windows 7 task scheduler, which is able to wake the computer to run a scheduled task. But clicking through all tasks is tedious, therefore I used my favourite tool, Windows Powershell:

get-scheduled-task –recurse | where { $_.definition.settings.waketorun }

That all. It produces a list of all tasks which are configured to wake the computer if necessary. I found the task, switched off the wake flag and now have to wait and see if that has been the culprit.