Urine Splash Backs
Impurity or najasat needs some moisture to transfer to some other body. If it is dried, it does not transfer. If one does not know that any part of his dress or any part of his body is involved with najasat, he is not answerable. He needs not to bother. There is no need to look for impurities, let them come to your notice, then you can remove them. Impurity in small amount is ignored.
If you really see a trace on your feet only then you should remove it.