My guess is that they had this vehicle info long before yesterday. It wouldn't surprise me if they had it 6 days ago. Some info like this they don't want to release because as soon as they do, the suspect knows and ditches that evidence. The suspect could now see that their vehicle info is out there and just dump the vehicle in a ditch to never be seen again. On the flip side, if the police don't reveal the vehicle info to the public but they keep an eye out themselves (ie alerting all officers in the area, or even the country), they might be able to locate the vehicle without spilling the beans.
Releasing info can be risky if the suspect is following the investigation.