Maybe the caller is not accusing anyone of anything. Imagine there's a stabbing. The police show up and immediately start questioning bystanders. No-one wants

Maybe the caller is not accusing anyone of anything. Imagine there's a stabbing. The police show up and immediately start questioning bystanders. No-one wants to talk. A detective hands out business cards, and says 'if you remember anything, then give me a call on my mobile'. On the way home, one of the bystanders sees a bloody cloth next to a public bin a block or two away that looks like the knife might be wrapped up in it. They're scared that it might have been a gangland killing, so they call up and report what they saw in confidence, fearing retribution. The caller's not made any accusation. The physical evidence stands for itself in court. There's no need for an illegal or warrantless search to find the knife, and neither does the tip-off form a basis for creating probable cause for a search. In this case, at the very least I think it is reasonable for the caller to expect a level of privacy, and the police should be obliged to inform the caller that the call is being recorded.