While scanning beneath the beacon band, the only low frequency radio
signal I didn't recognize was some data signal around 136Khz. I
detected it at night but neglected to see if it was present during the
day (which would rule out skip).

I didn't make the trip to basecamp, but noticed that the voice was no
longer present on their 209Khz beacon. The band was quite noisy, so
maybe I missed the voice, but I'm sure it was the right beacon since
AEC morse could be heard. Maybe they decided that shouting out
"basecamp" on the beacon at their inactive airbase was not very covert.