What should I do if I find a very small dry spot on my underwear, with no trace on the glans, and I suspect it came from the penis, or is it soap residue?

So long as you are in doubt about what you found on your garment—whether it is an impurity (hadath) or not—then nothing is required of you. This is because the default assumption is the absence of impurity, and doubt concerning the occurrence of impurity does not necessitate purification (taharah). Therefore, whoever is certain of their purity and doubts the occurrence of impurity, builds upon certainty.