To be honest, even if the radio remained on for two days, it does not draw
enough current to drain a car battery of any decent capacity.

Since you are electronically inclined, measure the current the battery is
drawing while off.  See if what the VW Garage is saying makes sense.  It
would have to draw more than an amp to significantly drain a battery in two
days.

If it is drawing that much current when off, makes sure no short in the
wires upstream of the radio.  If not disconnect the radio, again check the
drain on the battery.

If not drain, get rid of the radio, might me in warranty if a 2004.  Even so
aftermarket radios are often cheaper and better than originals.  Or go to a
boneyard and pick one up very cheap.

This should do it......let us know.

Joe

Put a amp meter on the battery and then pull the radio fuse.  If the
drain does not go down enough keep trying fuses one at a time until you find
the one with the big drain.  Keep in mind that a three year old battery may
just be hitting the end of its life.  You also should check out the voltage
off and the voltage when running.  It should be something like 12.5 off and
13.2 or more on.

You need to measure parisitic drain (that is the drain when the key is
off, doors closed, everything off that you would normally turn off).
You must let it sit, measuring the drain for at least an hour or so,
with periodic checks to see what the drain is. Chart these, make a
log, whatever.

Any drain over 70 MA (that's 0.070 amps) is considered to be high, but
you will probably find that there is an initial high drain that goes
away after about 5 to 15 minutes. This initial high drain is normal.

He has replaced the battery, hasn't he?

start with the easy stuff.

glove box light
trunk light
unplug the radio for a few days and see what happens.

Polo... I have no clue what they look like. Just not a USA vehicle.