  Increasing numbers of sub-stellar companions are now being discovered via
direct imaging. Orbital elements for some of these objects have been derived
using star--companion astrometry, and several of these appear to have
eccentricities significantly greater than zero. We show that stellar motion
caused by an undetected inner body may result in the companion elements derived
in such a way being incorrect, which could lead to an overestimation of the
eccentricity. The magnitude of this effect is quantified in several regimes and
we derive the maximum eccentricity error a third body could introduce in a
general form, which may be easily applied to any imaged system. Criteria for
identifying systems potentially susceptible to this scenario are presented, and
we find that around half of the planets/companion brown dwarfs currently imaged
could be liable to these errors when their orbital elements are derived. In
particular, this effect could be relevant for systems within 100 pc with
companions at >50 AU, if they also harbour an unseen $\sim$ 10 Jupiter mass
object at $\sim$ 10 AU. We use the Fomalhaut system as an example and show that
a 10% error could be induced on the planet's eccentricity by an observationally
allowed inner mass, which is similar in size to the current error from
astrometry.
