Is a tip probable cause for a search?

I've always wondered these things:

Suppose a neighbor claims to smell marijuana and calls the police. The police arrive, but there's no more smell. Is this probable cause for a search?

A neighbor or someone else suspects a grow op but cannot prove it and doesn't smell it. They call the police. Is this probable cause?

My instinct would tell me that it isn't, and a knock & talk is the best the police can do in either of the above circumstances.

as long as the cops dont smell it yur good,there wont be a search unless they obtain a warant,and yes it can be obtained in either cirumstance,but they have to have reasonable suspicion for that to occur.

tips are everything, no tips, almost no busts
if they get a tip, and they feel like following it up, there will be probable cause if they want it
depends a lot on your state/jurisdiction how they deal with a tip