Somebody Else's Problem field

The Somebody Else's Problem field, or SEP field, is a device from The Hitchhiker's Guide to the Galaxy that cloaks someone or something by making everyone view whoever or whatever is cloaked as somebody else's problem. They duly ignore the SEP field. Thus, for example, you can make whole mountains disappear, simply by painting them pink and erecting a small SEP field generator.

This handy gadget can be found in flash patches, in See-Through Devices, and can be activated in a PPC TARDIS if the chameleon circuits go haywire. It is not known if the SEP field turns the TARDIS pink. The Mary-Sue Factory used an SEP field to conceal itself, but was still discovered; it's possible the League of Mary Sue Factories still uses SEP fields for a similar purpose.

Breaking the SEP Field
The SEP field will cloak an agent from being detected by characters in a fic, unless:
 * The person observing them is not a canon character for their continuum (Mary Sues and OCs).
 * A canon character becomes so OOC that he suffers a character replacement.
 * A Sue notices the agent, and draws the canon character's attention to him
 * The agent draws attention to himself somehow. Talking won't do it, but a loud noise might, and interacting with the canon characters definitely will.
 * The character in question knows of the PPC's existence, or else is omniscient in some form.

A SEP field does not protect agents from being affected by events in the continuum. For example, it would stop Haldir from seeing PPC agents and shooting them; but it would not protect the same agents from being hit by a stray arrow during a battle, being drenched during a rainstorm, or being buried inside a collapsing building during an earthquake.

The SEP Field in Action
Usually, the SEP field works so well that nobody even mentions its existence. It is a credit to the DoSAT that most notable incidents involving SEP fields are the few times when they fail.
 * A Sue who noticed Agents Manx and Shadow, and pointed them out to the canons:
 * "Sue be Nimble, Sue be Fast, the PPC’s coming to kick your – "
 * Agents Laburnum and Foxglove laughing hysterically and knocking things over loudly enough to attract attention:
 * "Lullabies, Anal Rape and Dental Hygiene" (NSFW)