Hey, just a quick one as I'm on my phone: this is probably mad overkill but I can't help thinking that this is a good use case for many of the techniques mining exploration people use for mapping geological features (exploration geophysics).
It's not my personal focus but I think you'd have a good chance of finding the pipe using ground-penetrating radar or electrical resistance tomography (where I live the domestic sewer pipes are plastic but maybe you might have luck with a metal detector too). Some guys that I work with spent some time trying to use these to do realtime crevasse detection and I believe it's the same technique used for archaeological surveys. Whether it's a commercially available service with a reasonable price tag I'm not so sure.
Anyway as I said it's probably overkill but it immediately popped into my head so I thought I'd share. Cheers.