Thinking about it, even if conditions were right, I don't think that a 172 would produce visible contrails because the exhaust volume is just insufficient. Not sure that a 172 would produce a visible wing tip vortice either. Bit like looking for a urine trail from an ant!
I think the first answer about a knackered engine or a fuel additive to be the most likely reason and it would be visible smoke not water vapour!