Would a pen that was taken inadvertently, and whose owner could not be found, be considered a debt for which one will be held accountable on the Day of Judgment, and what should be done in this situation?

Indeed, Allah has overlooked for the Ummah of Muhammad (peace be upon him) errors, forgetfulness, and actions done under duress. Scholars have interpreted this to mean that this overlooking pertains to sin, but the rights of individuals are not abrogated thereby.

Whoever finds a lost item (Luqata) must return it to its owner, or to their heirs if the owner has died. If neither the owner nor their heirs can be found, then one should give it away in charity on their behalf, while guaranteeing its value. If the owner later appears, they are given the choice: either to ratify the charity and its reward be theirs, or to take compensation while the reward of the charity goes to the one who found and gave it away.