The other day, my car's front bumper slowly touched someone's car. I checked his car. It wasn't really scratched but had some paint stuck to it from my car. I went upstairs and then realized that I might have committed a sin. So I went back and noted down the number of the car. Now, I don't know this guy and I don't know if I should find him and tell him about the incident. If not, am I committing a sin?
Yes, as you said, you gave damage to someone else's property. No matter how little it is, there is a damage. I don't know if you could find the owner (I think you should put a paper with your number on the car), but at least you should try. And when you find him, you have to explain that it was an accident and you have to apologize. Then you should give the money required to fix it, or you may fix it properly.
This is counted as rightful due and Allah won't forgive this kind of sin.
And do not deprive people of their due and do not commit abuse on earth, spreading corruption. (Surat Ash-Shu`arā' 26/183)
[And Luqman said], "O my son, indeed if wrong should be the weight of a mustard seed and should be within a rock or [anywhere] in the heavens or in the earth, Allah will bring it forth. Indeed, Allah is Subtle and Acquainted.(Surat Luqmān 31/16)
EDIT for more Reference about Rightful Due
If you know Turkish, then I suggest that to watch this short video by Osman Ünlü. I am now going to summarize some of what he says about rightful due. He doesn't make comments, just explains and gives references.
He says that (below translation is belong to me, I hope I translated truely):
Allah-u Teala never forgives those who have given harm to the others, who has rights of others onto him/her unless the owners of the rights forgive. You have to get a word from the owner "My right is helâl to you, I forgive you." while the owner is still alive. If he/she is dead, then from his/her children/relatives. And you have to do a proper repentance after. Allah doesn't interfare to rightful due to forgive.
Allah doesn't give mercy to those who don't give mercy to other people. (Buhâri, Tevhid 2, Edeb 27; Müslim, Fedail 66, Tirmizi, Birr 16) Source
Those who had right of others onto him/her, who harmed the property and the purity of creatures, ask for owner's blessing and pay before die. Because in that day, there is no value of gold and property. In that day, his/her good deeds will be taken from him and if they run out, the owner's sins will be given to him/her until the right is fulfilled.
In the Day of Judgement, if the owner of the rightful due doesn't forgive his/her right, for a right as much as one dank (nearly a half-gram silver), 700 salat which is practiced with cemaat and accepted will be taken and given to the right's owner. (Ibni Âbidîn, Dürr-ül-muhtâr, Intention for Salat, p. 295)
The bankrupt one from my ummah is who will come with deeds such as salat, fast and zakat in the Day of Judgement. However, he/she has sweared to this and that, slandered, ate someone's property, shed blood of somebody and have beaten somebody. In Ahirah, his/her good deeds are distributed among right owners. When there would be no more good deeds, some of the sins of right owners's will be taken and shifted to the sinner. Then he/whe will be throwed to hell. (Müslim, Birr 6; Tirmizî, Kıyamet 2)