An SEP Field is used to hide something in plain sight. This phenomenon works on the principle that the human brain will filter out impossible objects and situations so as to preserve the sanity of the owner of the brain. When an impossible object or situation is encountered the brain decides that it is somebody else's problem and promptly deletes it from its perception of reality.

See Hitchhiker's Guide to the Galaxy.

It is known to cause neural disruption in otherwise normal people. Probably associated with a bogon field, but with further unexplained effects, like forcing high school students in strange rectangular patterns, or making otherwise reasonable people start to sing the national anthem.
The only way of protecting yourself from the SEP field is to stay well away from SEP, or wearing disreputable clothing. This will make the bureucrat perceive you as a completely useless person, and turn the withering blast of his field towards somebody else.
Another technique, an advanced one, is to square off the SEP man against a bogon sink, like an UNAM university professor. The result may well be a normal environment.

An SEP field is a device for invisibility, and history has an example of a real one:

In 1054, a star exploded. Actually, it exploded about 6000 years previous, but the light from the explosion only reached Earth in 1054. The expanding cloud of gas from this explosion is now known as the Crab Nebula, in the constellation Taurus.

At the time the light from this supernova reached Earth, the Catholic Church was the ruling political power in Europe, and their dogma, regarding any understanding of nature, was that good Christians must always defer to Aristotle, known as "The Philosopher" at the time, for any question about what was true.

This supernova was quite bright: It was visible by day for most of a month, and bright enough to read by it, at night. It was brighter than the full moon for almost two months.

This event was recorded by:

• the Mayans
• The Incas
• The Chinese
• The Australian Aborigines
• Many of the tribes of Africans
• Many other antique civilizations in Asia and the Americas

... which is why we know the year it happened.

Since heresy was such a serious, life-threatening charge, when European Christendom saw what must have been an unavoidably noticable light in the sky, they deferred to Aristotle, who had pronounced the Universe to be "eternal and unchanging."

Hence, this obvious change in the Universe was Somebody Else's Problem - everyone pretended like it wasn't there, rather than risk excommunication for heresy by talking or writing about it. It couldn't have been invisible - even clouds would not have masked its light. But everyone acted as though it was, and it's completely missing from European records: The Emperor's New Clothes, writ astronomically.

In 1731 this event finally made the history books in Europe, when the English amateur astronomer John Bevis first noticed it in his telescope. It later became the first celestial object listed in the catalog created by Charles Messier: M1

The brain of an animal (i.e. person) cannot possibly process every bit of sensory information that is coming to them at any given instant without an AMAZING waste of processing power.

Thus, only things that are immediately recognized as in some minute way relevant to survival, loss, or gain, or in civilized culture possibly something one would like to observe, actually are noticed.

Not an inherently sanity-perserving device, however maintaining sanity may well be a peripheral function of this tendency of the brain.

An S.E.P. field is an artificially created field of some type or another, projected on or around an object/event to obscure it. Works by triggering, or slipping under the tendency explained above, and causing an observer to simply not notice it, or mentally "write it off", for the same reason one doesn't notice that gravity is still working, or that one has bone's in ones body. Or less idealistically and more realistically, that the car across the street has its windows open and a storm is-a-brewin'.

In theory, one under the influence of certain types of Psychedelic drugs would be much less prone to be affected by an S.E.P. field.