[reverend_gonzo]

They may have information through a confidential informant (that they don't want to give up) or a method that is not legal (and can't be used to get a warrant). So, instead, they use a field test that may often show a false positive, but is enough to get the warrant and get inside.

[jandrese]

Confidential Informants like the cop writing down the license plates outside of the hydroponics shop?