Probable cause.
The police need a Search Warrant to enter a private residence in order to collect evidence unless they have reasonable cause.
They must provide a judge with probable cause to do a search.
Yes all police officers need a search warrany in the UK
Yes, the police do need a warrant to search your locked roof mounted cargo box.
I'll give an example. When police come knocking at your door, demanding you to open up and let them search the house, you can say, "No, I need to see your search warrant." The police MUST have a search warrant in order to search your house and if they don't, you can kick them out! :) Hehe.
A warrant may be necessary for the police to search the server that Facebook is running on. It is also possible that FB may choose to allow the search without a warrant. If the 'search' is simply examining publicly available information on any web page, no. A warrant would not be necessary.
Yes. The occupant/resident need not be present at the time the warrant is executed.
yes they have to have a paper signed by a judge
Yea
no they do not have to show you all they need is the search warrant. But what you should have done is to see the warrant. Sorry
they need a warrant
Yes. Depends upon the circumstances, for example, if the individual entered the residence to avoid apprehension and the police witnessed the act then they do not need a search warrant.