Frank Ocampo Gomez said:
So... Erin's technically asleep... yet the person heard Rad...
So is Erin imagining herself asleep and is really talking for Rad,
I'm going with this option.
Erin, in her slumber, heard the knock at the door, and is dreaming that Rad is up and about. When Rad said "Hello", that was actually Erin speaking (while asleep).
Note that the person behind the door said "Your voice sounds weird", and not "YOU'RE NOT ERIN!!". Because it *was* Erin herself speaking, but she was putting on a 'Rad-sounding' voice.
(I'm probably wayyy overthinking this.)