From The Urban Dead Wiki
Zombie spy is a term used to describe survivor characters who locate and watch safehouses on behalf of zombies, using flares or metagaming to announce their findings. In some cases, zombie spies tear down barricades, often using a Crowbar; after the May 31st 2007 changes to the game, this is now considerably more difficult to do discreetly, since anyone in the building at the time will be notified that someone is tearing down the barricades.
Attitudes to Spying
The general Urban Dead community is mostly split on the ethics regarding spying. Some agree that Zombie Spying is an acceptable tactic, although there are those who feel that it is not a legitimate strategy. Several of the larger hordes of Malton do not use these tactics as they are against it for Role Playing reasons or simply believe that it isn't fair play. Nowadays, is it common to see zombie groups against spying for this reason, however, many other groups engage in spying as retaliation for an unprecedented act of combat reviving on their character, making use of the character while they are in a survivor state.
There are many groups and individuals in Malton that maintain and follow lists of suspected zombie spies, and kill them on sight. The criteria for judging zombie spies varies by group, however, and results in non-spies being PKed often in some areas. Zombie spies used to be hunted down mercilessly and perhaps overzealously; over the past couple of years this practice has dwindled considerably, due to the game changes of PK displays in a building and the display of tearing down barricades from the inside.
However people still overzealously kill and encourage to kill dual natured, loyalists, life cultists, and regular players for minor or none provocation. E.g Frank Doyle (We are watching you Frank, stop killing us we are not zed spys.),
A few zombie groups such as After The Flesh and the Eastonwood Ferals openly admit to spying on safehouses for the benefit of either their own group or zombies as a whole. The Ars Requiem (while not officially admitting to the practice) even maintained an up to date map of the survivor status in East Becktown, with information about the barricades status and the number of survivors inside. This has been stopped since the group disbanded. Occasionally a feral zombie will utilize a human character for his or her own scouting, but close interaction of multiple characters is zerging and therefore such actions are outright cheating.
What's the opposite?
The opposite of a Zed Spy are called Loyalist Spies.