If found in public place either it can be given or may use it depends upon the person who go it. If the mobile is snatched from sombody it is an offence. If the lost person know the IMEI number, he/she can track with the help of the cellphone company and may get it with a request.
If someone had found a active Mobile with the SIM intact, then he must have contacted the owner and returned him that mobile. If the owner has not reported the matter to the police, then you are safe. But think of the situation, if some fraud has been committed through that Mobile and it is on police surveillance, then you will have to face the music. However, your act of not returning that Mobile to it's owner may not be a criminal act, but morally it is bad.