Approximately 2 years ago I found a screw bit (that fits on a battery powered screw driver) on one of the paths in our neighborhood. It is a good quality piece. I assumed that since it it is so small, the owner would have despaired of finding it, and I have since been using it. However, perhaps since it is of good quality the owner would have hoped to find it.
The issue here is that perhaps when you came upon the bit, the owner had not yet noticed that it was gone, and therefore had not yet despaired of it. On account of this concern, the obligation to return the item to its owner remains extant even after he has certainly despaired of finding it (after two years!). However, there is no way, of course, of finding the owner now. Therefore, you should write down the value of the screw bit on a paper, and keep it with your documents. If and when the owner will be found (“when Eliyahu will come”), you would return the value to its owner. This procedure would permit you to continue to use the bit.
Sources: The idea of writing down the value is adviced by Iggros Moshe (Choshen Mishpat vol. 2, no. 45), and has been adopted by other authorities.