They don't need a warrant for anything which was on you at the time of your arrest. That's not to say they can hold it forever or indefinitely *unless* it is or provides evidence of a crime, was usd in a crime (such as to arrange for drug purchases or sales), or is the "fruit" of a crime (such as having been paid for with illegal funds)--and in these cases, they can pretty much hold it forever--but certainly, they can hold it for at least several days or weeks during the investigation.