Start at the meter, not the faucet
Find your water meter — usually near the street in a concrete box, or in the basement/crawlspace access on older homes. Make sure nothing in the house is using water: no dishwasher, no sprinklers, no ice maker running. Watch the small leak-indicator dial (it's often a small triangle or star on newer meters). If it's spinning with everything off, water is moving somewhere in your system whether you can see it or not.
Narrow it down before you call anyone
Shut off the valve under a toilet or at your main shutoff and watch the meter again over 15 minutes. If it stops moving, the leak is somewhere between the meter and that shutoff point — inside the house. If it keeps moving with the main off, the leak is on the service line between the meter and the house, which is a different kind of job and usually a bigger deal. Either way, you now know which conversation to have when you call a plumber, instead of guessing.
What actually causes a spike like this
The usual suspects, in order of how often we see them: a running toilet flapper that's silently leaking (can waste 200+ gallons a day and you'd never hear it), a slab leak under a concrete foundation (often shows up as a warm spot on the floor if it's the hot line), or a failing supply line to a fixture. None of these announce themselves loudly. That's exactly why the bill is usually the first real signal homeowners get.
When to stop DIY-ing it
If your meter test confirms movement and you can't visually spot standing water anywhere, stop opening walls and call someone with actual leak-detection equipment. Acoustic and thermal tools can usually pinpoint a leak's location before anyone cuts into drywall or flooring — which means you're paying to fix the leak, not paying to search for it.