How old is the battery first and foremost.
Check any door and or trunk pin switches as they can fail and sometimes allow the battery to drain by leaving an interior light on.
Most times its the trunk one and thats why people can't find it. Can't tell the trunk light is on when the trunk is closed.
I hope that helps you get started on tracking down this problem. A Dying battery can be annoying.
Have you had the charging system checked?
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.