Anyone else just (15th July 2015) started seeing Apple's captive portal probe popup in 10.9.5? It may be a sneaky silent push (this isn't affecting 10.10.x) due to Flash security issues?
We've worked around this by pushing out (we use Munki) this to our devices and asking the user to reboot.
/usr/libexec/PlistBuddy -c "Add ProbeParameters:SSIDExceptions:**SSIDName**:ProbeProtected bool true"