Connecting and disconnecting the battery in a modern car will just about
always result in small sparks, clicking relays etc.

Your say you have 5mA current drain with everything off.  This looks a
little low but is possibly accounted fo by the multimeters realtive high
restistance when measuring low currents.

If your are sure you have no current drains, e.g. the boot light is
soemtimes common cause of unexplained battery drain, then you could be
looking at battery self drain.  This happens when sediment, which builds up
over time, effectively shorts the bottom of the battery plates out resulting
in internal current drain.

In my experience if this internal drain is happening then a battery left
disconnected over night will most prabably survive.  However when connected
the natural car idle load will push the battery too far.

If the battery can withstand 100+ hours disconnected and still be able to
start the car then a vehicle electrical fault may be present.

Often the easiest solution is to replace the battery and see what happens.
Many battery outlets will test your current battery free of charge.  The
problem here is that  when you have driven to them the battery is quite well
rejuvinated by the drive and my well pass their tests.  If you can take just
the battery, after 24+ hours of standing alone, then the tests results are
far more reliable.

Nick /////