What is the ruling for someone who finds a small trace of a wound with a little blood on one of the limbs of ablution after completing it, and does not know when it occurred, and does this affect the validity of the ablution or not?

The fundamental principle is that every incident is attributed to the nearest time when its occurrence is probable. If the questioner does not know when the wound occurred, then it is presumed to have happened after ablution (wudu’). The questioner is advised to disregard obsessive thoughts (waswasah).