  The Transit Timing Variations (TTVs) technique provides a powerful tool to
detect additional planets in transiting exoplanetary systems. In this paper we
show how transiting planets with significant TTVs can be systematically missed,
or cataloged as false positives, by current transit search algorithms, unless
they are in multi-transit systems. If the period of the TTVs, P_TTV, is longer
than the time baseline of the observations and its amplitude, A_TTV, is larger
than the timing precision limit of the data, transiting planet candidates are
still detected, but with incorrect ephemerides. Therefore, they will be
discarded during follow-up. When P_TTV is shorter than the time baseline of the
observations and A_TTV is sufficiently large, constant period search algorithms
find an average period for the system, which results in altered transit
durations and depths in the folded light curves. Those candidates can get
subsequently discarded as eclipsing binaries, grazing eclipses, or blends.
Also, for large enough A_TTVs, the transits can get fully occulted by the
photometric dispersion of the light curves. These detection biases could
explain the observed statistical differences between the frequency of multiple
systems among planets detected via other techniques and those detected via
transits. We suggest that new transit search algorithms allowing for
non-constant period planets should be implemented.
