That isn't really how it works. The search warrant must be narrowly scoped to only what is required to investigate the crime for which they have probable cause.

That isn't really how it works. The search warrant must be narrowly scoped to only what is required to investigate the crime for which they have probable cause. However if they find evidence of other crimes while staying within the scope of the original warrant they can generally use that evidence at trial. So for instance if the police have probable cause that a wanted criminal is at your house they can get a search warrant to search it for him. If while searching they see illegal drugs on your coffee table they can use that evidence under the plain view exception. However if they open up your schoolbag and find drugs they would not be able to use that as evidence because there is no way a person can fit inside your bag so searching it couldn't have been part of the search authorized by the warrant.