To repost the geocaching forum:
http://forums.groundspeak.com/GC/index.php?showtopic=25352
I went ahead and looked for the broken and unbroken X's in Central Iowa and found a few, too. Fred&Edna, these geocashers are calling them "unmonumenteds" (noun)?
Using GoogleEarth "fly" over at about a 3k elevation. These sets of symbols can be easily found in the fields at:
Burns Rd. and Tiffan Ave., Marshalltown, IA
Parker Ave. and US 30 Bypass, Marshalltown, IA
Comments made on the geocacher forum quite interesting. I think they are calling them unmonumenteds. They may be a reincarnation of the old skull and crossbones symbols, who knows? The geocacher member mentions recategorizing these sets into "Skull"--wonder if that is a techy term or if he is simply joking.