What constititues reasonable suspicion to pull someone over?

The Fourth Amendment to the United States Constitution protects citizens from unlawful investigatory stops of their vehicles by peace officers.[1] Investigatory stops of vehicles are only permissible if the peace officer has a reasonable suspicion that the driver has been, is, or is about to be engaged in criminal activity. Reasonable suspicion is dependent upon both the content of information possessed by the peace officer and its degree of reliability. Both of these factors are taken into account by when evaluating whether the peace officer had reasonable suspicion to stop a vehicle.

That said, from the facts you have provided, I can hear the prosecutor arguing that this was a welfare check. 

            A community caretaker vehicle stop allows a peace officer to stop a vehicle based upon his concern for the driver’s safety.  Usually, a peace officer is only justified in stopping vehicles when: (1) The peace officer observes the driver of the vehicle commit a traffic violation; (2) the peace officer has reasonable articulable suspicion that the driver is committing a traffic offense, such as driving under the influence of alcohol or driving without a license; and (3) the peace officer has reasonable articulable suspicion that the driver is engaged in more serious criminal activity, such as transporting drugs.

            The Utah and United States Constitutions prohibit peace officers from randomly or arbitrarily stopping vehicles.[1]  However, Utah Courts allow vehicle stops if the peace officer is genuinely engaged in a community caretaking function. 

In order for the community caretaker vehicle stop to be lawful, a Utah Court must find that the peace officer complied with a three-tiered test.  First, the court must determine if a seizure occurred under the Fourth Amendment of the United States Constitution. (Stopping an automobile and detaining its occupants constitutes a seizure under the Fourth Amendment.[2]) Second, the court must determine whether the seizure was in pursuit of a bona fide community caretaker function.  Third, the court must ascertain whether there was a reasonable belief that the circumstances posed an imminent danger to life or limb.

Hope this helps.