I was driving past Lindberg Field yesterday, and glanced to my right as traffic flew in, and this little bird caught my eye. That’s a DASH 7 airliner converted to a US Army EO-5C Airborne Reconnaissance Low- Multifunction spyplane. The EO-5C performs both image and signal intelligence functions in support of anti-drug operations in Columbia, operations in South Korea, and has been purported to have supported operations in Afghanistan.
They aren’t secret or classified (though details of their operations probably are) but they are rather discrete, with non-standard markings. I spotted one at John Wayne Airport a few years ago.