how do i find plastic lateral lines for septic tanks that use no gravel?

I know of no place that the code does not call for gravel the gravel is added for drainage of the liquid you will have to have and inspection and I do not think it will pass with out it

The system is called a "chamber" or "bio-diffuser". Works better than gravel because of the amount of free space left for the liquid to be absorbed into the ground. There is no way that I'm aware of to trace it without first adding a tracer wire in the ditch before you backfill. (Some cities require one) You may try your county enviromental office. They are supposed to make a scale drawing of the site when it was installed, if it was permitted in the first place.