They should have been charged or released by now:
"The police can hold you for up to 24 hours before they have to charge you with a crime or release you.
They can apply to hold you for up to 36 or 96 hours if you’re suspected of a serious crime, eg murder.
You can be held without charge for up to 14 days If you’re arrested under the Terrorism Act."
https://www.gov.uk/arrested-your-rights/how-long-you-can-be-held-in-custody
I think the police must have requested an extension but they're not publicising this as it would then be clear that they no longer think Becky is alive. I think they're trying to make it appear that this is still a missing person enquiry because someone who hasn't been arrested knows something and they're watching them and hoping they might slip up... JIMO