Sand found in bathroom drain via vent pipe

[Greg Wmson]

Tenant complains about overflows and standing water in the back bathroom toilet, so I snake out that vent 50' in and it comes out clean with no real blockage, so I snaked out the bathroom in the middle of the house on the other side of the problem bathroom wall and while doing so, I found sand at the bottom of it, from my guess about 4-6 feet from the ground level of the vent and could not go any further, and almost got stuck a couple of times upon pulling it out I found sand caked silt/sand in the auger head., I thought I'd better call a real plumber and have him video it for me.
Any chance this is NOT a broke main pipe under my slab?

History. in the past about 5 months ago the city's connections caved in so it's possible that sand got in the system but that was 50' away and downgrade, so I'm not convinced that's the problem. Plumber videoed the pipe and that's how they found the collapsed city pipe, I'm just wondering how they did not find this problem in the process.

 

[Sylvan]

This is why I like to snake then water Jet and then do a video inspection ($1,500)
The snaking makes sure there is no roots rags or other hard stoppages

The Jetting scours the piping that sludge ,soil ,grease may cover up defects and the video which I charge $1,000 hopefully shows ay piping defects

If someone does just a video they many miss something and think it is normal waste that adhere to the waste line