That's an odd one. My guess is that someone improperly did some invasive investigation of the core or someone improperly did a repair. What was wrong in either case is that when they put it back together, they did not grind out a 12:1 scarf on each side of the cut. Assuming that the outer skin is 1/8" there, that would mean that they should have ground out 12/8" (1.5") of a ramp on either side of the cut and laid in cloth and biax with different widths until the ground out area was filled. Without that treatment, any flex or stress that is applied to the hull would have cracked there at the weakest link.
As Dave said, the repair is fairly straightforward. I also agree that the $145 for an Electrophysics EP-33 or similar meter is a good investment, but as any good surveyor would tell you, there is more to it than that. Wet core isn't good and will lead to future problems if not addressed, however, the more important information to glean would be if the core is no longer holding the inner and outer skins at a fixed distance. This would be because either the core has turned to mush or the skins have delaminated from the core, which would likely be the result of a very wet core and numerous freeze thaw cycles. The tool to use here is a small ball peen hammer to listen for a good sharp report in the response to light tapping or a bad dull sound.
Good luck,
-Chris
