I saw that too because one of the cats jumped into the window that was left open. I remember at the time because I thought that it must be a sloppy crime scene if they're allowing pets to come wandering in. About 30 seconds later it was back outside again.
Until they had evidence ,the Police would not have been able to treat the house as a crime scene.
I think the Police could have been initially treating it as a case similar to the Shannon Matthews kidnap and therefore hoped Tia would be found alive.