The police do what they want.
When it comes to money to burden of proof is on you. You better be able to show that it was obtained through legitimate means.
Even so.. many times they'll make you fight for it through the legal system. It can sometimes be cost prohibitive to try to obtain your money back... and that is provided that they even acknowledge the full amount they took from you.
Locked briefcase and in the trunk of a vehicle. This eliminates SUVs, trucks, hatchbacks, etc. You want that shit in a clean vehicle, locked case, in a locked trunk.
..and as always. Never consent to a search of your vehicle. Ever.
Cops can and do... whatever the fuck the feel like.
they only need to suspect that it is illegal proceeds, suspicion is not a high hurdle to clear..