What is incumbent upon one who concealed two bicycles with the intention of stealing them, then could not find them, nor does he know their owner, who may have been martyred, given that he did not take them and someone else may have stolen them?

Sincere repentance erases sins, and through it, Allah transforms evil deeds into good ones. It is achieved by abandoning the sin, regretting it, and resolving not to return to it, while also returning rights to their rightful owners. If you took the two bicycles to steal them and they were lost, you are liable for them and must return their value to their owners or their heirs. If that is not possible, then donate their value on their behalf. If you do not know their exact value, then estimate an amount that you are reasonably certain covers their value or more.