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I built houses for a living for 4 years and build apartments now. Assuming you're using a typical concrete anchor that only requires 3-4" of embedment/penetration, there is zero chance you will come anywhere even relatively close to that toilet sewer line. All foundation penetrations go at least a foot or two straight down before turning towards the main. Engineers try to avoid having horizontal drain pipes cross foundation trenches because the chance of soil shifting damaging pipes drastically increases. What year was your home built? Post tension didnt come about until the mid-90s so depending upon your local soil conditions, there are a few different ways the foundation could be built. The sewer systems shouldnt vary too much though. If that spot isnt over a structural beam on the slab, which it probably isnt, then the concrete is only ~4" thick there anyway.
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I deal a lot with remodels, I sell 40ish a year. Unfortunately, it's impossible to know if they were done under the watchful eye of big brother, or if the simplest solution meant sloping right after the flange.
I nearly typed out a similar statement to yours, but I thought it would be misleading because I see a ton of evidence on a yearly basis that it isn't always true. My experience may be specifically based on remodels (whether improper or proper), but it is significant enough to me to give me cause to at least extend a warning.
The good news is, if you hit the sewer line, it's not a difficult fix, just time consuming. lol.