Barney's cat reference?

[Thelaststory]

I just got past the sniper part of "Follow Freeman!" and as Barney's opening the gate, he turns and asks you if you hear a cat, and then mentions that it haunts him.

What is that in reference too? Is it something further along in the game (don't tell me if so, this is my first play through) or is it from HL1 and something I missed or have forgotten.

 

[Polaris]

Back in the Kleiner's lab (Red Letter Day) Barney said "You mean it's working? For real this time? Because... I still have nightmares about that cat."

 

[Thelaststory]

Oh yeah! Haha...I thought it sounded familiar, I just couldn't place it.

Makes you wonder...

 

[TectonInd]

What cat?

 

[MuToiD_MaN]

What cat?

Thanks, Alyx.