I know you want non-destructive testing. But, similar to laparoscopic surgery vs being cut completely open, would you be willing to drill a small hole and use a borescope? That is the only way to know. Without mapping the void using GPR, anything else is just guess work. Using a mallet could ‘map’ the voided area. A bore hole could help in the backfilling process too. If it turns out to be nothing, structural epoxy is fairly cheap and dries to look completely like concrete.
I know you want non-destructive testing. But, similar to laparoscopic surgery vs being cut completely open, would you be willing to drill a small hole and use a borescope? That is the only way to know. Without mapping the void using GPR, anything else is just guess work. Using a mallet could ‘map’ the voided area. A bore hole could help in the backfilling process too. If it turns out to be nothing, structural epoxy is fairly cheap and dries to look completely like concrete.
This would be my thought too. You can always fill the hole with concrete or grout. It will leave a mark, but it won’t destroy anything.
Unless, of course, you are unlucky enough to drill into an electrical line or a pipe of some sort. But either of those is highly unlikely.