Warning Sign

FIG. 1 is a front view of a warning sign showing the claimed design;

FIG. 2 is a rear view thereof;

FIG. 3 is a top view thereof; and,

FIG. 4 is a right side view thereof.

The left view is omitted because it is identical with the right view; the bottom view is omitted because it is identical with the top view; the dot-dash broken lines define the boundaries of the claimed design and form no part of the claimed design; any other broken lines illustrate portions of the warning sign and form no part of the claimed design.