Is it permissible for me to benefit from the sack of rice I found on the seashore, considering that it might have fallen from a car, and there is no one in this remote area whom I can ask about it, and I took it fearing its spoilage?

The found rice must be kept for one year because it does not spoil quickly. If there is concern of its spoilage before that, it is permissible to use it while ensuring a similar amount is given to its owner or by donating it on their behalf. When the owner appears, they are given the choice between its value and its remuneration. This is based on the Prophet's (peace be upon him) hadith regarding found items. As for trivial things that have no customary value, the finder may use them.