When can law enforcement collect evidence without a warrant?

An officer of the law can only search for evidence if they have reasonable cause in doing so. All persons do however have the right to see a warrant before allowing a search, which is their right. All officers are aware of this, however, and if asked will produce a warrant. Without this warrant the evidence may be inadmissible in the court of law.

Law Enforcement does not have to show you the warrantprior to serving it, or arresting you. Simply the knowledge that the warrant exists is sufficient for them to conduct their activity.

They must, however, leave a copy of a search warrant at the residence, if it was a search warrant for a home, and an inventory of things taken, if any.