Battery being run down to flat '98 C220 CDI

[noelmcs]

Starter motor replaced on car about 1 month ago. Car started perfectly for 2 days. On day 3 battery completely flat - not enough energy to power ignition lights. Battery replaced - same problem again in a couple of days. Brought car to main dealer mechanic on Friday last - June 19th. He has now put heavy duty battery in car. I dont think the source of leak has been traced & that heavy duty battery is just buying time. i am assuming he checked the alternator. Car has alarm fitted since new. Parrot device for hands free phone fitted about 5 months ago. I would appreciate any advice comments.