Some of these would make great oldschool spy- or detective-movie props, like the rubber stamp being overlooked by "stupid" detectives whilst the smart detective directly connect it to a callsign to uncover the murder victims secret life with regards to ham operations.

I've seen morse code written around the border of images (e.g. Spy vs Spy did this) which easily goes ignored by the uninitiated, and even when noticed can often escape further detection.

Another note: since in the US there are only four letters allowed as the first letter (A, K, N, or W) and will be 4-6 characters with further restrictions on which can be letters and which can be numbers (2-by-3 being the most common, 1-by-2 and 2-by-1 only being available to higher license classes, where those are the numbers of letters before and after the region identifying number). You could use this in your detective story, along with other details like inferring where they got their license from or getting more details from the FCC. Following this up with an inspection of the radio and which frequency was recently used, could make for some interesting detective work. Ham Radio doesn't make very many appearances as a plot device.