What to do when you find an unclaimed (lost) property:
The first thing you have to do if an unclaimed object comes in your custody is make a public announcement. Then wait for a year to see if the owner returns. If he doesn't you can utilize it.
Sahih Al-Bukhari Volume 3, Book 42, Hadith 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." ...continued.
Is picking it up lawful?
continued... The Bedouin 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."
This part of the hadith explains two cases:
The 'Sheep' which is potentially harmable. The sentence "It is for you, for your brother (i.e. its owner), or for the wolf." means that till the owner finds it, if you don't guard it, the wolf will have its meal. The sheep here can be compared to any fallen object not abled (designed) to protect itself eg. a book, cash, cell phone, etc. and the wolf can be compared to any harm that can be caused. So you should pick it up, make a public announcement and guard it for a year. If the owner still doesn't turn up, you can keep it for yourself.
The 'Camel' which can protect and feed itself. The camel can be compared to any potentially un-harmable object eg. a stone slab etc. These things should not be picked up.
ۚ وَاللَّهُ عَلِيمٌ حَكِيمٌ (And Allah is the All-Knower, the Wise)
اللَّهُمَّ انْفَعْنِي بِمَا عَلَّمْـتَنِي وَ عَلِّمْنِي مَا يَنْفَعُنِي