data being used in a phone that isn’t switched on?

Hi, I’ve worked in phones for 12 years and I haven’t got a clue what is going on. I bought a 1 month rolling sim for my son for his birthday but I haven’t actually set the phone up yet. The data is showing that 50% of the data has been used?? This is quite literally impossible the sim has not been used at all bar when I quickly got the text to set up the app. I have no clue what is happening and it’s his birthday on Friday 🙃 please help