Is a person sinful for not informing a security guard that he was the one who caused a water pipe to break and a little water to leak, after he guided him to the location of the break for repair?

If this pipeline, installed with the permission of the ruler, is damaged by your passing vehicle, you are liable for what you have damaged. You must refer to the authority responsible for it. They may oblige you to repair it, or pay the cost of its repair, or they may pardon you. In cases of damage, there is no distinction between intentional and accidental acts, and your informing the security officer does not absolve you of liability.