I have recently opened another bank account with a competitor and have also recently moved home. I transferred about £1500 from my Santander account to my other account online.
A few days later I had trouble signing into my online account so I contacted customer services who advised me that they suspected a fraudulent transaction on my account and so i had to answear a few questions to get my account up and running again. They advised me that they spotted the £1500 but couldn't stop it. I received a letter a few days later saying that they couldn't stop the £1500.
Luckily, it was going into my own account so there was no concern, but Santander noticed it but didn't do anything about it.
Further to this, I can't believe in this day and age that it still takes at least 2 working days to do a balance transfer with Santander online!