Triangle printed on outlet

[lgmagone]

I am sitting in the library at the University of Washington and am noticing that the outlets in this room have a small triangle printed on them. The triangles are directly above the left prong of the upper outlet. The outlets are 20 amp outlets.

What does this triangle mean?

Is it part of the NEC code?

 

[480sparky]

Isolated ground.

406.2(D).