Should one re-pray missed prayers after discovering dried semen traces on clothes, given that the discovery was made days after a wet dream without noticing the semen at the time?

It appears that the semen you saw was the result of a wet dream, and you must perform ghusl (ritual bath) and repeat the prayers you performed after the last sleep in that garment. However, if you find a sign indicating that the wet dream occurred before that, you should repeat the prayers from the time indicated by that sign. This is what the jurists have stipulated; Ibn Qudamah said: "If one sees semen on his garment, and no one else sleeps in it, he must perform ghusl because it is not possible for it to be from anyone but him. He should repeat the prayers from the last sleep he slept in it, unless he sees a sign indicating that it was before that, in which case he repeats from the earliest sleep from which it is possible for it to be."