What has not been mentioned by the previous posters suggesting HF is that nowadays huge chunks of HF spectrum can be recorded using SDR and played back at leisure, with particular attention to transmissions sticking out as unusual. Likewise VHF, if anyone is recording it in that way.
This is interesting because intelligence networks like GCHQ do something along these lines and as the aircraft was equipped with something like Rockwell Collins HF 400W transceivers which can work any frequency between 2 and 30Mhz then it is possible those on the cockpit had capability of communicating on preassigned frequencies with a range of 1000s of miles.
The key things here are 'if it stands out' and 'if it was heard'. Short HF transmissions in the middle of an Amateur Radio or Maritime frequency block are, I suggest, highly unlikely to draw attention to themselves unless there is something spectacularly unusual about them, even assuming that the signal is received loud and clear at a monitoring station. And even if someone spotted them immediately and identified the transmissions as unusual, unless the messages contained clear intent and identifiable place names, there is going to be very little that can be done about it. Something like "Alfa this is Bravo, all according to plan, ETA position Charlie four hours" isn't going to give much away unless you can recognise the speaker's voice or instantly triangulate it (good luck with that) to a location of high interest.
The radio bands are cluttered with all manner of transmissions from any number of sources, most of which are going to be unidentifiable unless they choose to identify themselves. Ships, aircraft, expeditions, taxis, radio hams, smugglers, take your pick ...