I don't think there is anything specific other than the discovery of some evidence indicating foul play which could be almost anything depending on the circumstances. If they had found blood in his car, for example, or some of his clothing discarded with or without blood, or his phone was found somewhere it shouldn't be.
Thanks for the reply. Does this mean if something is found or has to be changed to a murder enquiry? As in it's out of anyone's hands it is just changed?