After p-trap, waste pipes goes level for 6 inches, then drops about 3 inches before going into walls, under bathroom sink. Is that a danger of s-trap siphoning if large volume goes down sink drain? Is this an S-trap? Or is it safe because of the level 6 inches of pipe?

If the Ptrap doesn't connect into a vent then yes, you have an Strap there. To resolve this you could connect a mechanical vent (called an AAV.. air admittance valve) just after the Ptrap and that would fix the issue.

Mark

And here's how it looks,(see image) Good luck, Tom

Thank you very much.