Friendly reminder that in Jeff's early concepts he would possess a tentacle about popping out of his head. Although it is unknown if it would have a use, such as attacking or something, we can assume that like the tentacle and Jeff share the same sensory characteristics they could help each other when it comes to detecting the enemy.
In the art you posted it kind of looks like the hydra tentacles from the HL2 Beta.
True, it bears an immense resemblance, but within the notes of these conceptual arts, information about those blue tentacles has never been provided.
He listens to sounds just like the tentacle so maybe Jeff was just thought up as a walking tentacle at first
UUUH, funny way of looking at it. As Jeff mixed with the Xenian environment, a large part of his being (in this concept art) would be a tentacle, speaking in the sensory part.