Battery goes flat!

98 DeVille.  Onshut down, everything turns off, as far as I can tell.  Yet, on occasion, I find the battery completely discharged and am unable to start until battery charger brings it up again.
Cadillac service can find no reason for it.  Alternator replaced short time ago as was Battery.  This has happened 4 times recently.  ??????   Everyone is at a loss as to cause ?????  Any suggestions?   Gene Hamilton --  Indianapolis, IN 

Make sure the trunk light is operating properly, which means turning off when the trunk is closed. Simple, but make sure battery connections are clean and tight.

Finally CUT the wire to the trunk light to make SURE it was unlit.  Battery still discharged and had to have a 2-hour battery charger lift again.  The battery voltage was down to 10.1 volts.  ???????? gene hamilton

How old is the battery?

Well, here is what I do to test for a battery discharge while the vehicle is sitting and not running. Remove the negative battery cable and hook a test light between the cable and the battery post. If there is any "draw" on the battery, the test light will light, although it will be very dim for a minor draw, and very bright for a significant draw. If the test light glows even dimly, you have a draw on the battery. just watch the test light and remove all of the fuses 1 at a time till the test light goes out. If the light goes out when you remove a certain fuse, than you will know that the draw is in that circuit. This will help you to narrow down the circut that is causing the draw.

try the cig-lighter socket maybe loose

Yes good idea and cell phone chargers are notorious for drawing down the battery, always make sure it is disconnected when car is not running.