I guess OS seismic Ground-based Nuclear Explosion Monitoring (GNEM) is possible. Here's a high-level pdf slideshow about it and machine-learning processing. [0] Matlab-based MatSeis exists and the code part is FOSS. I guess it would take a moderate amount of software and hardware engineering effort to make a working, turn-key FOSS GNEM seismic detector. It seems prescient given the decline of warhead, missile, and test ban treaties.
Furthermore, in the proliferation arena, it's possible to map breeder reactors to rough regions using terrestrial antineutrino mapping like KamLAND and what was Borexino. These aren't really conducive to open source initiatives because of their civil engineering burdens of scale (unless one has a mine or a missile silo) and so need to be state/academic efforts until some clever engineering might make them possible (don't see how it's possible because of physics) on the surface and at smaller scales.
0. https://www.osti.gov/servlets/purl/1582524