Most commonly the cause of a toilet randomly running for a few seconds is the flapper in the toilet tank that lets water out of the toilet tank into the toilet bowl.
When you push down on your toilet flush handle it lifts a flapper which allows the water to leave the tank and go into the toilet bowl to flush the toilet.
Sometimes that chain gets kinked just a little bit but enough to not allow the flapper to seat fully and seal off the water from entering the bowl.
Try checking the chain for kinks or adjustments and if that's not the problem then try holding your hand down on the flapper to make it seal better to see if any water leaves the tank and if the flapper seals better while you're pressing down on it then the flapper is bad and needs replaced.
They're cheap and easy to replace and all you need to do is shut the water off to your toilet, flush the water out of the tank and then remove the flapper and take the flapper to the hardware store with you to get a correct replacement and then just snap the new flapper back on and attach the chain to it and adjust the chain on the flapper so that the flapper in the toilet seals properly.
Toilet tank flappers usually only last around 5 to 6 years and it's a good idea to replace the flapper at 5 years because it's much cheaper than the water bill would be if the toilet leaks water out.