You can find a hidden $500 money stash in the corn fields in the Sunset Motel mission.
It is worth mentioning that you can pick this up every time you do the mission, even if you do not complete it. So technically you can go there, pick it up and leave and you'll still get the $500.

Start the mission and go straight ahead. Go past the motel.


You'll see a gap in the corn fields on the right. Follow it until you reach the suitcase.



That's it. There is or will be more hidden stashes in the game probably but so far this is the only one I have found. Will try to update accordingly with new locations.