Once I got sent to locate an empty conduit under a parking lot. I was given a fish tape. Theoretically this should work, but it didn't. I knew it wouldn't but I don't question the infinite wisdom of my employers.(only intimidate them and call them stupid later). No ground, unconductive fish tape.. who knows.
Then the piece of shit broke on me and it was impossible to put back together. The storm/sani lines in B.C. can be as shallow as 2-4ft.

Does the windshield jug/tennis ball method really work? Don't they get stuck?
No experience w/GPR....... but I've seen it...honestly.

We have single mode fibre from our handholes to the bldg without sheath. It is designed this way. For this reason a tracer is installed in the conduit for location. Every now and then there is a locate to be done where for whatever reason(laziness) they have not installed a tracer. One of our contract locators ran into this problem. I went to trouble shoot, and we located the conduit w/cable using the rodder and a reel of tracer. I believe it was Nazi who stated the signal should dissipate towards the end, or more accurately will only travel so far. Nothing knew to anyone old school, this might work from the house to the main, never tried. A sonde unit would be better. All the locating companies I ever worked for were tooo(yes,.3 o's) cheap to buy sonde units, while still claiming to have the ability to perform such work(usually with shitty old 810's...ok) Plans are always best for mains. Or you can just witch it... that's how i do ALL my locates!