Listen. If you hear the tank refilling every few minutes and nobody flushed, the flapper is not sealing or the fill valve is not shutting. Both are common, and both are cheaper than hunting a slab leak that is not there.
A flapper, fill valve, or flush-valve repair typically runs $150-$400. A full toilet replacement, when the bowl is cracked or the flange is rotten, typically runs $350-$900 installed. A toilet that rocks has a failed wax ring. Water at the base after a flush, not a constant drip from the supply, is that ring. Left alone it rots the subfloor or the flange.
Dye in the tank is the home test. If color shows up in the bowl without a flush, the flapper is leaking. If the fill valve hisses and the water level sits at the top of the overflow tube, the valve or the float is the problem.
Replace the supply line if it is the old plastic kind while the toilet is apart. Those lines are a flood waiting on a brittle nut. We will match you with a plumber. Ask them to say which part failed before they quote a new toilet.