I don't think I would worry about 28ma. Draws caused by trunk bulbs etc. usually range around 200ma. I would guess that the spec for that car would be 25-50ma.
With a diesel I would try and monitor the glow system. If the system comes on or stays on intermittantly it will keep the battery week. Often a hard onee to catch.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician