"And this was the safe route?" Gordon wonders aloud. "Hold on- did you hear something chirping?"

It's the first of several hopper mines. As much as Gordon would like to make good use of the things, he doubts anyone's going to be chasing them any time soon, so the best he can really do is grab one up with the gravity gun and punt it as hard as he can in the direction of the gunfire. Moments later, he's rewarded with the sound of zombie parts hitting the wall- and maybe some Combine as well.