Weird experience on train last week

Friends were on a train home last week in the late afternoon, police were on the train asking for go cards. They didn't have card readers with them...litterally just checking if you HAD a go card or ticket.

My friends tapped on with their debit cards so the policeman asked to see the bank statement on their phone? They felt(understandably) uncomfortable to do so, so argued the fee might not even come through until the next day. The policeman agreed and moved on.

Any one else expereince this? Was it reasonable to ask for a bank statement? I think I would feel pretty uncomfortable and a little suspicious?