Pre slab inspection is an overview of preparation works by expert in the hope of discovering items of significance that have been missed by others before him.
It is like a bird's eye "big picture" view rather than detailed on the ground checklist analysis. Typically people engrossed with detail can miss an elephant sneaking up behind them.
Typically it is review of siting, setbacks earthworks and levels. Then formwork, set out lines, drainage and other services. Depth and position of trenches or stiffening beams, special features, size position, tying lapping and clearance of reinforcing and overall position of plumbing pipes termite protection etc. Then have the plans been followed?
There is a lot to know and a lot to look at
Its the expertise in knowing from experience what can go wrong and then having a nose to find it.
This is my comment in recent post where plumbing pipes were in the wrong spot:
"What happened to you is a perfect argument for pre slab inspection by your own inspector. In your case
1 plumber
2 SS
3 Builder's QA
4 Building Inspector
5 You
All missed it
Yes it is not going to be pretty and your slab may be weakened and will become a patched job.
What else has your builder missed?"
It's also putting builder on notice. Hey we have an expert looking after us.
So, I suppose, the important question is, in your example, would you have picked up on the pipe in the wrong spot by taking a "a bird's eye "big picture" view rather than detailed on the ground checklist analysis".
I completely agree that one can spot many problems with a slab with a "birds eye view" and experience is absolutely invaluable but to be able to, as you describe your process, "Typically it is review of siting, setbacks earthworks and levels. Then formwork, set out lines, drainage and other services. Depth and position of trenches or stiffening beams, special features, size position, tying lapping and clearance of reinforcing and overall position of plumbing pipes termite protection etc. Then have the plans been followed?", without taking levels and detailed measurements, then it would be very difficult to give a definitive answer that every aspect of the siting, slab (including slab thickness) and plumbing is correct.
While I would advocate strongly to the average home builder, with little or no experience, to use an independent building inspector, they also need to be aware that an hour on site and without complete measurements and levels being checked, they cannot expect all issues to be noted.
If I was paying for a pre slab inspection (probably the most important of all) I would rather pay double the fee and have all of these details thoroughly checked.