Early during his stay on prehistoric Earth, Ford Prefect receives a faint and distant signal through his Sub-Etha Sens-o-matic:
Nevertheless, Ford Prefect began to use his Sub-Etha Sens-O-Matic more often again. Only once did he pick up a signal, but that was so faint and from such enormous distance that it depressed him more than the silence that had otherwise continued unbroken.
~ The Restaurant at the End of the Universe
Is the origin of this signal ever adequately explored?