OK I was using the toilet and I heard the water come on ffor like 2 seconds.
This has all new fluidmaster flush valve and flapper w/i the last 2 months.
What would cause this

flapper leaking by ??
put food coloring into the tank to see if it shows up in the bowl..

I don't think so because for the water to drop low enough for the float to kick the water back on, it would have to run longer to raise the float enough to shut off..
I shut the water off and marked the level...I'll check when I get home

You said it did this "while you were using the toilet". If that means you were sitting on it at the time then maybe you leaned back against the tank or some other posture that temporarily let water from the tank go into the bowl? If so it won't happen again until you or someone sits and duplicates the event. Most toilet installs has a space behind the tank over to the wall, if you lean back on the toilet hard enough you can cause the tank to tilt backwards and put strain on the mounting between tank and bowl and the water supply line if it is rigid. At my parents house I added a thick sponge (like one used to wash a car) between the tank and the wall to help keep the tank from tilting back.

[edited to fix spelling]



Edited 5 times.

no I was standing ....and my tank touches the wall...nowI turned the water off at 8am and rechecked just now 2pm...s 6 hrs,,,never moved

so now that you have marked the water level, turn the water back on and let the toilet do its mystery 3 second filling. check back in a few hours to see if the water level went up.