Bit hard to tell what the photo contains .. however the flaps leave a fairly strong trailing vortex and in conditions of moderate to high humidity the core will be seen due to condensation.
A characteristic of any vortex is a significant radial pressure gradient. The pressure reduction is associated with an adiabatic temperature drop .. the RH increases and at dew point you have a visible record of the vortex core in the condensate.
This is probably what you were seeing, I suspect ?
[ 23 February 2002: Message edited by: john_tullamarine ]</p>