Might of been from the TTC. The test had been done a few years before, but It seems possible that a test item got washed down to a river and out to sea. 200 inches of rain per year on that side of Panama.
The Sisters is pretty far from any serious run off. The thing was pretty far from neutral buoyancy as well. I have always wondered if it was a private test or some larger entity. I figured it was private at the time because Panama did not even have LORAN back then so once placed finding it again would have been tough.
Any way, if anyone reading this DID the test, the reason you can’t find it is I took it. Just to let you know, it FAILED. Which surprised me, actually. The container did not. It was like the saltwater actually penetrated the PVC itself. No way was the ammo shoot able.