Long story short, I'm a trained screener/identifier with a FD job. Went on a regular med call in an apartment complex and while waling in, my pocket Radiacode went off. Didn't think too much of it as I was staying dedicated to our call for service but once the medic left to transport I went back to the same area to verify the hit.
It hit again. Moved away, readings defreased, moved closer, readings increased. Turned away (using body as shield), readings decreased. Broke out the monitors from our apparatus (Ludlums) and confirmed presence with secondary equipment and notified our special ops guys (they carry the expensive stuff.)
Narrowed it to a single apartment, requested PD as a precaution, did a knock-and-talk with the source apartment occupants and found that about 4 hours earlier in the day one of the apartment residents had done a nuclear stress test.
I was really stoked to have found my first source in the wild and very much relieved that the source was legimate and of no nefarious use.