Presumably this is a diesel and I guess that you're not driving it very far on each journey. If this is correct, then the DPF will not be passively regenerating and will seek to actively regenerate by introducing diesel fuel via the engine to the DPF to burn off the soot in it. If you switch off the engine while this is happening, that diesel will sink down the cylinder bores into the sump, raising the sump level. So, whenever you stop, open a door before you switch off the engine and, if you are aware of intense heat under the car, keep driving it for another 5-10 miles to ensure active regeneration is completed. I know this is inconvenient, but it is the penalty of running a diesel car with a DPF for short distances. Next time, buy one with a petrol engine.