My PCO friends experienced that..
He visits 1 month interval for rat control.
He observes the rodenticide is eaten up within 1 month but can't identify when it was eaten up within 1 month.
Every month he saw such thing.
He knows the target rats is not resitance trait.
Then he wonders even if rats eat up the enough dosage of rodenticide, most of them recover and keep alive when there is certai days without rodenticide (they may be eating other healthy food).
I wondered at first they got resitance ability but he didn't agree.