Legal Question in Constitutional Law in Iowa

Not necessarily. Warrantless home searches are often perfectly legal. This most frequently happens when officers enter with permission from someone who has, or appears to have, authority to grant it. Police also don't need a warrant to conduct a search incident to a lawful arrest, if they have a good-faith belief that someone in the house is in immediate danger, if they are in hot pursuit of a fleeing suspect (yes, the rule really says "hot pursuit"), or in certain other situations.

Even officers who have a search warrant typically aren't required to show it. That is why they're able to conduct searches when no one is home or when someone seems to be home but nobody comes to the door. (Requiring officers come back later or to wait until someone returns would create opportunities to thwart investigations, and would often cause serious delays -- sometimes in situations where time is of the essence.)

Some jurisdictions may require officers who have a warrant to show it only if asked. Others don't require it even then. I don't know what Iowa law says about this. Even where police are supposed to voluntarily show the warrant, there will be exceptions. I doubt that any U.S. jurisdiction requires officers who do show a warrant to wait until the person has finished reading it before searching.

Courts sometimes issue "no-knock" warrants when there is reason to believe either that an officer would be in danger if she announced her presence or that someone inside the house would destroy crucial evidence (flushing drugs down the toilet, for example) upon learning the police have arrived. Some states may require them to show the no-knock warrant if asked once they have entered, but you asked only what they're required to do beforehand. Except in unusual situations (e.g. if the suspects happen to be in the front yard when police arrive and then decide to cooperate), there is no way to show a no-knock warrant before entering the home.

One last point: Even if the police aren't required to show a warrant, you're allowed to ask them to. They will often agree.

I hope this info is helpful.