It can take several weeks for a dental crown to pass thru the digestive system. Its moving much slower than your food. Look at a human digestive tract diagram on the internet, the small and large intestines are very long and it can take a long time for it to pass thru. You can go to your nearest store or hobby shop that sells metal detectors for gold prospectors. They can use a simple device to pinpoint exactly where it is, with your clothes on. If the crown is metal based, covered in white porcelain or gold its easily pinpointed. If its a plastic crown then this will not work. Be patient, use a strainer