Auto topup was switched on which of course was topping up £10 each time your credit droped below £3.
Have the agents confirmed that it was data doing this? Was the usage they showed higher than what was included in your goodybag?
Alternatively this credit may have been used by premium rate calls or texts, if you have not made any of those yourself, not are aware of receiving any premium text service, then the next thing to check would be your installed apps, as many will ask for permission to access your phone/contacts/messaging so if you was to have a scam app installed (normally a game you have downloaded) it may send premium texts in the background or make premium rate calls, both of these would go to a number owned by the scammers, and hence is a easy way of them stealing your money.
Please check what apps you have installed recently.