tilled soil will show up as hot(from what i have read) especially when worked over and ammendments have been made. i dont know if it is movie lore, and it might be. that if you can find a fresh field and lay in it they couldnt see your heat outline. a few scattered plants shouldnt show up. i think it was a vietnam movie i watched now that i think about it.
and yes police still need warrants but if the have probable cause such as seeing a pipe or drugs or weapons, something along that line they have a right to search without a warrant. no we didnt vote on that congress or some other higher power made that decission.