[peterwwillis]

https://webcache.googleusercontent.com/search?q=cache:wZ0Cex...

http://www.gocsc.com/userfiles/file/ortronics/whitepapergovt...

https://security.stackexchange.com/questions/10447/is-the-us...

I'm honestly not familiar with these kinds of facilities, but with a potential "Collateral Confidential" type cable, one could imagine a gas-filled tube being a countermeasure of some sort.

[jessaustin]

For instance if the gas were poisonous?

[inopinatus]

More likely there is a pressure sensor, intended to detect splicing attempts.