All Praise to Allah Subhanahu wa Taala and blessings on Prophet Muhammad peace be upon him His family and companions
According to Hanafi Fiqh,
If someone assumes about something or act that this has not happened and he swears that this has'nt happened but after that later on he came to know that actually "it did happen " (Suppose you went outside from your home at that moment you saw your brother in the home now someone asks about your brother and you swear that he is in the home but later on you came to know that he is not) then it is called Yameen laghw and for Yameen laghw there is no Kaffara,so he does'nt need to do that as asked in the question.
From The Source:
An oath is a verbal statement conjoined with a phrase of swearing, such as “By Allah” or “I swear by Allah.”
There are three types of oaths:
(1) The Engulfing Oath (yamin ghamus), which is to swear by Allah that one did or did not do something in the past, while deliberately lying about it. This is a major sin, so enormous that no expiation is due, since expiation is not enough to lift the sin. Rather, one must make sincere repentance and seek Allah’s forgiveness.
(2) The Enacted Oath (yamin mun`aqida), which is to swear to do or not do something in the future. If one breaks this type of oath, expiation is due.
The expiation is to feed ten poor persons (2 meals each), or to clothe them (1 garment each), or to give them the equivalent monetary value. If one is unable to do that, one must fast three consecutive days.
(3) The Mistaken Oath (yamin laghw), which is to swear about something in the past, thinking that it is as such, while in reality it is not as such. No expiation is due, and we hope that Allah will not take the person to account for it.
A vow is to verbally swear that if something happens, one will do some act of worship, like praying, fasting, giving charity and the like. If that thing does take place, then it is mandatory (wajib) for him to fulfill the vow by doing that act of worship.
For more carification or Fatwa contact the Mufti.
Allah Swt and His Messenger knows best.