From a lawyer's website:
Exigent Circumstances. This exception refers to emergency situations where the process of getting a valid search warrant could compromise public safety or could lead to a loss of evidence. This encompasses instances of "hot pursuit" in which a suspect is about to escape. A recent California Supreme Court decision ruled that police may enter a DUI suspect's home without a warrant on the basis of the theory that important evidence, namely the suspect's blood alcohol level, may be lost otherwise.
So what should you do if the police show up at your house "just wanting to look around?" It's not in your best interest to deny them access because there may be extenuating circumstances that you don't know about; you certainly don't want to risk physical injury or being charged with interfering with a police investigation when you didn't have anything to hide in the first place.
However, do make it clear that you are not consenting to the search. Ask the officers for identification and an explanation as to why they are there and what they're looking for. Also, write down details of the search as soon as possible, in case you need them later.
Dave S