...at least I have only seen the Army do it...

Occasionally, I'll see soldiers that will have two white rectangular tabs on the back of their helmet.

Why?

There are two small reflective rectangles sewn on the green elastic band that goes around the helmet.  They are called cat's eyes.  They can be used for visibility on patrol or when a squad leader needs to control a dispersed formation.  You might sometimes seen them sewn onto the back of soft BDU caps, especially in certain units; i.e. Rangers.

If you don't want them to show you can just roll the band over so they point in.

Thanks!

They're also called
"Ranger Eyes".

Believe me, it is HARD to see them in the dark if there is little or no light to reflect off the tabs.