You must have been online and didn't see my other post. If it happens when no one is using water, then yours sounds like the toilet ballcock. Those things can leak and go off at any time.
Sounds like time to R & R your toilets and especially if nothing has been done to them in 10 years. Last problem I had, I just replaced them all cause I figured another one will be going bad shortly. Replace the flapper balls also.
Go to the hardware store and get a fluidmaster that matches your brand of toilet and flapper. You may want to take the flapper with you. Out in the country you could have a pump problem but I'd start with the toilets. Good luck.