If you had to say something without alerting others, codewords are the best way to do it. So use something that everyone knows about. What would you use?
Weeping Angels – someone that you can't take your eyes off
Daleks – destroyers, assassin
Cybermen – gangs that want everyone on their side, they will kill anyone that doesn't join them
The Master – crazy person, but they're brilliant, major problem
Waters of Mars (Gas Mask Zombies) – infectious, don't touch anything related
Red-Eyed Ood – they're being controlled by someone else
Archangel Network – something that connects everyone
The Doctor – someone you can trust without question
Companion – someone that goes around with the Doctor or the Master
Bad Wolf, Blaidd Drwg (variations) – warning, bad, major trouble
TARDIS – major transportation that would get the person almost anywhere
Vortex Manipulator – minor transportation, can't get everywhere
Sonic Screwdriver – something that gets the person into anywhere
Laser Screwdriver(may not need to be said) – gun, weapon
Gallifrey – no longer exists
Time lock – something that you cannot get into, no matter what
Fez, Bow tie – trendy items, it's cool at the moment
UNIT – government, some type of official agency
Torchwood – organization that no one knows about (supposedly)
Time Vortex – major power
Graham Norton – someone that interrupts at the worst possible time
Pandorica – holding cell, prison
Rassilon – Someone that was good, now completely bad, off rocker
Harold Saxon – someone that seems good, rotten on the inside, politician, upstanding citizen
Captain Jack Harkness – flirt, player
White point diamond – something that comes from nothing, something from Gallifrey