If it is infrequent then it may relate to DPF regeneration cycle. The temperature of the DPF goes way up to over 600°C so if there is any oil or fuel around it will smoke off as the temp rises. If there are no fuel pipe leaks then there may be an exhaust leak at some point in front of the DPF you will get burnt diesel smell and probably smoke out of the engine compartment. Its really difficult to spot. Have your mechanic check the exhaust joints for evidence of carbon (a leak).
Does the service history show any turbo or EGR or other work carried out?