A bomb like this could be pretty deep though. They're pretty aerodynamic and heavy yet compact. They'd go through mud pretty easily. If it's a few metres deep I doubt it would show up on a metal detector. This is why so many are still found.
There's now ground penetrating radar that archeologists use to find chambers deep in pyramids. I doubt it was around when the runway was built, but now there's clearly a case to use it.
