"A wash of some indescribable emotion," Erich thinks, then immediately describes the look on Biggles' face as "something like sleepy delight." But obviously it's just too painful if it's really that, so: indescribable! Mysterious!

Right???

Erich: his face bore some incomprehensible emotion
The Narrator: pleasure. It's pleasure. He's happy to see you, you walnut.

... Though to be fair, he IS getting some mixed signals here. But mostly he's just being an emotionally constipated, self-loathing idiot (affectionate) as usual.