Here is another set of symbols to think about. Mine and demining symbols.
http://www.imsma.ch/index.php?id=1226
It really depends on how WWIII starts in the US, but the Army could lay large amounts of mines to protect the northen and southern borders. A recone unit could run into them.
Here is the finial report on them:
http://www.gichd.org/fileadmin/pdf/I...inalReport.pdf