A warrant is issued by the court for failing to appear or breach of bail. Its also possible to be 'wanted' by the police without a court warrant being issued. This would be if he was suspected of having committed an offence but the police had been able to find him to arrest and interview him, or if he had been on police bail(called 47/3) and had failed to answer this bail.
It is possible that he became 'wanted' between Sunday and now, if the court date came and went, he missed police bail, or was circulated as wanted by the police. Also things take a while to filter down, be placed on the computer etc.
If he is wanted on a court warrant he must be put before the next available court when he is arrested. There are no courts on Sunday so he would be locked up for the weekend. His best bet would be to hand himself in early (before 8am) on Monday morning - but you don't have to tell him that!!!