During an inspection of the master bedroom. I was running the shower watching it drain when the toilet “tank flush valve pipe” discharged water over the top of the pipe. flapper never moved. I will always lift the tank lid to inspect for damage, and look at the age of the parts. This toilet was just changed out, old one still in the garage and looked original to the home.
I asked my customers if they saw the same thing and they said they heard it more than saw it. I’m already have the ? mark flag up. so I repeat the same thing and this time I watch the toilet not the shower.(its on and running) and sure enough we all see it discharge water into the pipe and down the drain. about three seconds worth.
talked with home owner and they both have hearing aids, but the wife as noticed it once in a while.
Both were told to have the original installer come look. There is no leaking at the time and toilet has been in for two weeks. BUT, I COULD NOT EXPLAIN IT TO MY CUSTOMER. Even as a licensed Contractor, I could not tell them what was the cause or fix.
(after 20min) had to move on, couldn’t get it to run again after the first two times.
Anyone else see this or have a plumbers point of view.
Thank you
Mike M
Mike, I haven’t seen that. And you can report what you saw/heard and refer it out to a qualified plumber for further assessment/repair. 
P.S. It is nice to see you back on the forum. Enjoy!
If you’re referring to the overfill pipe/drain inside the tank, water will go in there if the flush valve isn’t adjusted correctly to shut the fill water before it starts going over into the overfill pipe and or the flush valve isn’t shutting off as it should. Regardless, just have a plumber fix it, simple 
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Sounds like a bad fill valve. It sticks open and tank water runs into the overflow until the valve closes. Could be intermittent but will still waste water. Those fill valves are cheaply constructed even on new toilets.
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