It's entirely possible if flying at conditions of high humidity and temperature around the dewpoint for the pressure drop within the wingtip trailing vortices to cause visible fine condensation to drop out. If you want to call this a contrail, be my guest!
I'd struggle to find the reference, but recall reading somewhere that this was a problem suffered during WW2 by the photo-reconnaisance Spitfires, etc. over Germany - if you weren't careful to stay out of the band of temperature / altitude / humidity where this could occur you were at risk of broadcasting your existence to half of Germany.
G