Yes. It is usually referred to as 'plain view.'
To come into a home the police need a search warrant to search. Without the warrant the evidence is not admissible in court. It would be an illegal search.
A Search Warrant
Well, if the police department thinks that a cell phone may hold important evidence, then there is no need for a warrant. By the time the warrant is issued, the evidence may have been already destroyed or deleted.
Carroll Doctrine
They must provide a judge with probable cause to do a search.
No they can't. If they try to use it in court it would be immiscible.
Yes they can search it
The police need a Search Warrant to enter a private residence in order to collect evidence unless they have reasonable cause.
Yes, the officer may search. Police may search a building if they reasonably believe a valid search warrant has been issued. They do not have to possess the search warrant.
The police got a search warrent in order to search the suspect's house. The suspect would not allow the police to search his home without a search warrant.
No. If the search warrant is invalid and they illegally searched you or someone you know (like without reasonable cause, or without the warrant) then they can't use any evidence against you. To the best of my knowledge, anyways. I don't know how many ways a search warrant can be wrong though...if they were searching for like, pot, but found cocaine, they CAN use that though. Or a gun, or something like that. If they have the warrant, they can use it. If they searched without the warrant, its invalid and inadmissible 100%.
In the UK, the Police and Criminal Evidence Act 1984 provides the framework outlining the powers of the police, including the cases when a warrantless search can be conducted, and when a warrant is required.