Someone forgot a pen at one of the computer desks in the library i was was using…for a long time noone came back so i took that pen with me - Sin? even when i know no one comes back looking for a cheap ball point.
Re: Question.
:salam2:
Hope these two hadith clear out ur point 
In Sahih Bukhari Book ‘Lost Things Picked up by someone’ Hadith No. 608.
Narrated Ubai bin Ka’b: I found a purse containing one hundred Diners. So I went to the Prophet (and informed him about it), he said, “Make public announcement about it for one year” I did so, but nobody turned up to claim it, so I again went to the Prophet who said, “Make public announcement for another year.” I did, but none turned up to claim it. I went to him for the third time and he said, “Keep the container and the string which is used for its tying and count the money it contains and if its owner comes, give it to him; otherwise, utilize it.” The sub-narrator Salama said, "I met him (Suwaid, another sub-narrator) in Mecca and he said, ‘I don’t know whether Ubai made the announcement for three years or just one year.’ "
Hadith No. 609
Narrated Zaid bin Khalid Al-Juhani: A bedouin went to the Prophet and asked him about picking up a lost thing. The Prophet said, “Make public announcement about it for one year. Remember the description of its container and the string with which it is tied; and if somebody comes and claims it and describes it correctly, (give it to him); otherwise, utilize it.” He said, “O Allah’s Apostle! What about a lost sheep?” The Prophet said, “It is for you, for your brother (i.e. its owner), or for the wolf.” He further asked, “What about a lost camel?” On that the face of the Prophet became red (with anger) and said, “You have nothing to do with it, as it has its feet, its water reserve and can reach places of water and drink, and eat trees.”
Re: Question.
You are right that no one comes back looking for cheap disposable ball pen. If you feel guilty , give sadaqa/charity for the price of pen .
Re: Question.
Is it theft or is it making good use? Was it taken with a view to steal? Was it taken with a view to use? You can leave a note in the location that you found it. "I found a pen here, if you want it back, etc"
Or as brother mirch suggested.
If you fear sin then say "astaghfirullah" just in case.
Re: Question.
I started writing with it because i didn't want to go in my bag to find my pencil case etc...after writing with it i just put it in my notes and took it with me since i was at that place for about 5 hours and no one came to take it - I just used it as a bookmark...