The Fourth Amendment to the United States Constitution protects Illinois residents and everyone in the country from unreasonable searches and seizures. Under the court-interpreted meaning of the amendment, this means that police cannot conduct a search or seize someone's property without a warrant except for very specific circumstances that the court has carved out.
One of these circumstances is when the person who is subject to the search has voluntarily consented. This rule is easiest to interpret when only one person was subject to a search or when the property that was searched was one and inhabited by one person person. However, the reality is more complex than that and in many cases police search an area that is owned or lived in by more than one individual.