No, law enforcement does not always need a warrant to seize property. A police officer can seize property if he or she is in the process of arresting a person or conducting a lawful search, or if the officer conducts a search with your permission.
However, if property is seized without a warrant, the officer must have “probable cause” to make the search that leads to the seizure. This means that before being searched by an officer, there must be facts that would lead a reasonable person to think that a crime had been or was about to be committed.
In our example, Officer Potts did not have a warrant, but he would argue that he had "probable cause" to search the car and seize the cash because he believed he smelled marijuana. Because Officer Potts believed he smelled marijuana, it was reasonable for him to believe that the crime of illicit drug use had just happened in Steve’s car. Alternatively, if Officer Potts asked Steve for permission to search the car and Steve said yes, Officer Potts could legally search the car and seize Steve’s money.