I have a galaxy s3 (samsung) and all of a sudden it has started to switch on wifi on startup.
Despite the fact I turned the wifi off on the previous day before switching off.

Dont all shout at once!
lol

Have you got Cerberus anti theft or any other app you think that could control the WiFi? I have Cerberus and find that it turns WiFi on for a couple of minutes at startup if the SIM card isn't authorized with the app.

Dependant on location settings, isnt wifi switched on for this??