Search warrants allow you to search an area that might reasonably contain the items listed in the warrant. Once they are in the safe deposit box, it sounds as if they are using civil asset forfeiture to steal the money.

Not long ago people were joking on Steve Lehto's YouTube videos that the police might as well do forfeiture on armored cars. In California, the police started doing exactly that. You can hear them lamenting in one case that they only seized $300,000 and not the $1,000,000 they were expecting. This practice is being fought but it is still common in the US for police to seize assets just because they feel the assets might have been used in a crime, or might be proceeds of a crime. No criminal charges are necessary, and you have to sue to get assets back.