The woman could have contacted the rat poison from many areas......
1) when she came to wash her hand, she contaminated the tap key by opening it, so when she closed the tap after washing her hand, her hand is no longer clean.
2) if the woman has long nails, there is a possibility that some of the rat poison stuck on nails even after washing her hands.
3) THE SOAP. She contaminated the soap when she first touched it, so when she collected the soap again after washing i.e the soap is now wet, she contacted the rat poison again
4) when she went back into the house, she could carried the poison fish to the designated area, if she did that she could contacted the poison again......