First of all, Yes, an area of origin can be your entire building.

NFPA 921-2014
"18.8 Origin Insufficiently Defined. There are occasions when
it is not possible to form a testable hypothesis defining an area
that is useful for identifying potential causes. The goal of origin
investigation is to identify the precise location where the
fire began. In practice, the investigator has an origin hypothesis
when first arriving at a fire scene. The origin is the scene.
Sometimes, it is not possible to find an area or volume that is
any smaller than the entire scene. Thus, a conclusion of the
origin investigation can be the identification of a volume of
space too large to identify causal factors, or where no practical
boundaries can be established around the volume of the origin.
An example of such an origin can be a building that has
been totally burned, with no eyewitnesses. Such fires are sometimes
called total burns. The area of origin is the building, but
in reality no further testable origin hypothesis can be developed
because there is insufficient reliable data."

Secondly, even as the primary opponent of FIBFP, the first scenario that popped into my head when reading your description was the same as Sir Gary.

Check sections 18.8 et. seq. of NFPA 921-2014

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]