By the way, it is not at all easy for this to be sewage getting into your water lines, but it might be a problem of siphoning back into your house lines from the toilet tank itself. If so, it is still a bad thing that really must be repaired. I like the idea of a couple of drops of food coloring as a test of this. It might happen only when the toilet is flushed, and the faucet for the sink is also open.
If that test shows nothing, then redo the test, but this time, turn off the water pressure into your home at the main supply. (There will be a shutoff valve in your basement.) Now, open the valves in your downstairs faucets. This will create a siphoning effect, trying to suck water back from the toilet supply tank.
Since this has happened only once, my guess is it happened when the water pressure for your home was turned off. This allowed water to siphon back into the water supply lines from the toilet tank. An old valve in the tank might explain that.
Regardless, if you confirm this is what happens, then I would add an anti-siphon valve (also known as a back-flow prevention valve) into the water line to the tank. This is a spring loaded one-way valve, that allows water to flow into the tank, but not the other way.
Could this be a copper corrosion issue as the plumber claimed? This seems unlikely for that to have happened since you have not seen it before, but anything is possible. If you have that much copper in your water that is leaching out of the supply lines, this would be something to worry about. So if you do the food coloring test, and there is no sign of backflow from the tank, then I would get a water test done for copper. In fact, a quick check on Amazon finds a home water test kit that includes a test for copper in your water.
I would say to wait and see if the water continues to come in after a few days. It is quite possible that the sprinkler water is seeping through above the slab.
Are there bathrooms or other water usage utilities above where the leak is? Pipes usually rise vertically from the ground to the bathroom.
You could also try to shut off the main water valve for as long as possible to see if the water ingres shows down. Switch off water heaters before you do this.
You need to adjust the sprinkler heads so that they are not spraying on the house.
Best Answer
Have you checked your gutters. Every time I end up with water in my basement it is the result of some issue with my gutters. I have a lot of old trees around my house and during certain times of the year it is impossible to keep them clean (I know I need guards). If they back up the water pours over the sides and then builds up near the basement walls and eventually seeps inside.
Also, make sure the downspouts are all still attached and if needed get some extensions to send the water further away from the house.
Also, check the grade of the area surrounding the foundation. Make sure that there is a least a small slope away from the house. If the rock bed is too high - or if you have some kind or plastic underneath to prevent weeds it might be preventing the water from draining properly.