  Difference imaging provides a new way to discover gravitationally lensed
quasars because few non-lensed sources will show spatially extended, time
variable flux. We test the method on lens candidates in the Sloan Digital Sky
Survey (SDSS) Supernova Survey region from the SDSS Quasar Lens Search (SQLS)
and their surrounding fields. Starting from 20768 sources, including 49 SDSS
quasars and 36 candidate lenses/lensed images, we find that 21 sources
including 15 SDSS QSOs and 7 candidate lenses/lensed images are non-periodic
variable sources. We can measure the spatial structure of the variable flux for
18 of these sources and identify only one as a non-point source. This source
does not display the compelling spatial structure of the variable flux of known
lensed quasars, so we reject it as a lens candidate. None of the lens
candidates from the SQLS survive our cuts. Given our effective survey area of
order 0.71 square degrees, this indicates a false positive rate of order one
per square degree for themethod. The fraction of quasars not found to be
variable and the false positive rate should both fall if we analyze the full,
later data releases for the SDSS fields. While application of the method to the
SDSS is limited by the resolution, depth, and sampling of the survey, several
future surveys such as Pan-STARRS, LSST, and SNAP will avoid these limitations.
