How old is the building? Is the piping actually embedded into concrete?

It's quite possible that if it is, and it's been there for years, you've got cracks/holes in the piping itself, either caused by alkaline conditions in the concrete damaging the copper, or possibly thermal expansion and contraction fatiguing it. Plus there is a known issue with copper pipe developing pinhole leaks over the years for unknown reasons.

If this is happening it might be that your missing gas is leaking inside the concrete and soaking into the pores of the stuff, which could quite possibly absorb a remarkably large amount of it over time. It would probably eventually make its way to the outside and evaporate, likely over a large area making the concentration low. Although 700 litres of the stuff is rather a large amount to misplace.

As the smell is not inherent to the gas itself, LPG (which in your part of the world is probably pure propane) being odourless, it's also feasible that the ethyl mercaptan odourant is being filtered out by the concrete, I suspect. If this is indeed the case the only way you'd find the leak would be with a flammable gas detector and a lot of time and effort.

Re-piping it is probably the best solution.

That said, are you certain someone didn't install a sneaky tap-off point and run it into an adjacent building when it was built?

pca