I've been behind quite a number of recent (i.e. under 4 years old) Ford TDCi models that have smoked like billy-oh under relatively mild acceleration, in daylight conditions, and the odd VW, too.
My TiD with the Steinbauer P-Box only leaves a visible trail at night, and then only when trying hard. It's all down to the current condition of the car more than anything else, but I'd say if it's leaving visible smoke when warmed up, under moderate throttle and in daylight, it needs looking at. The rest of the time....it's being a diesel.
(Citroen HDi engines seem to be much better than most, until you chip them....)