There are several aspects to this.
- Nuclear Test Ban Safeguards
To detect the unauthorized testing of nuclear weapons, there is a global network of monitoring stations. These would have detected and pinpointed a crash site if it was severe enough, along with every meteorite impact of any size. Only really big signals are looked at. If there was some reason to go back and look at the data, it would probably be there. So, by this method, never unless there's something extra.
Probably wouldn't see anything. Even if something was seen, it would be ruled an artefact (technically correctly) as it's not what they're interested in.
- The Southern Hemisphere
As we know from assorted missing aircraft and lost expeditions throughout history, the southern hemisphere is big, mostly empty and mostly devoid of monitoring. And if a kakapo saw anything, they're wise enough not to talk about it.
So here's a theory your character might come up with:
If the ship crashed in the southern hemisphere near the ring of fire, nobody would see it, no radar would pick it up, most ground-based detectors would either be confused by the tremors from the ring of fire or programmed to filter any such tremors out, nobody would stumble across the wreckage and even if they did there are only a small number of societies the outside world would take seriously.
If you can be 100% certain that no system has that data (recognized or not), that's one of the few ways it can be done.
If you think a system may have recorded it but left the information unprocessed, you have a broader scope. Eastern Europe is secretive and paranoid, so an explosion is likely to go uninvestigated. It'll be picked up on seismographs, but if it's in an area with illicit fireworks factories or suspected burial sites for excess WW2 munitions, scientists would write it off as an accident and won't look.