nice -- no lets say GREAT  job on providing vehicle info-   my 89 was
a little stubborn getting the level to stay between it's hot / cold
levels. After a few tries, it  did , it has been fine for over a year.
Have you looked for things like damp rugs with the coolant smell?
looked  for fine mist sprays while the engine is warm and running?
i take it you 've watched for puddles after it been sitting.
if you don't find any evidence of coolant remains you're looking at
a internal to the engine leak -  could be a failed head gasket or a
crack in the head.  does the exhaust smell sweet? White smoke
out of the exhaust more than the usual cold damp morning smoke?
oil in the coolant?  coolant in the oil?

Does your system hold pressure?  If not you might want to replace the cap
anyway.

Look at the bottom of the water pump carefully.  Might be a leak only when
the engine is running but it should also leave residue by the shaft or the
weep hole if applicable.

I had to replace the head gasket on my '83 1.8L engine.  There was a small
crack that allowed coolant to slightly leak out of the front of the engine
by the timing belt.  Leak eventually started to get worse.

Yours might also have a plastic coolant outlet (or two of them) on the
cylinder head.  These sometimes deteriorate, change to an oval shape on the
inside and start to leak.  You might have to pressurize the cooling system
to get things to leak.
Should be easy to do with a tire valve stem, bicycle pump, hose, bolt and a
few small hose clamps.  Disconnect the little hose to the coolant bottle and
install the tire valve stem inside of the hose.  You might have to trim the
stem to fit leaving it somewhat larger on the end you insert in the hose and
clamp it tight.  Then use some hose to fit the coolant bottle nipple, stick
a bolt in the hose and clamp both areas.
(The fat section of the valve stem properly clamp should keep it inside of
the hose.
Then just pump up the bicycle pump after you connect it.  You might be
pumping for awhile!  <g>
If you develop too much pressure the cap should kick any excess pressure
out.

To release pressure hold the valve stem up high and push in the valve.  You
are trying to release pressure without getting a shower of coolant.  ;-)

My 1991 Golf was consuming coolant regularly starting a year or two
ago.

Didn't notice any leak at first so I assumed it was a bad head gasket.
Then I saw coolant dripping heavily from the water pump one day in a
parking lot, about 6 months ago.  That's a frightening scene.

After I replaced the water pump I was back to consuming just a little
coolant, so I thought...yeah water pump was definitely bad, but I
probably still need a head gasket.

Not seeing any drips again until a month ago when I noticed tiny wet
marks on the driveway under the radiator.  Could easily have been
condensation dripping down in those areas.  Then I noticed the stains
getting bigger.

Yep, bad radiator.  Was tough to spot without the driveway stains,
because it was probably doing most of the leaking when the engine was
hot and the cooling system under pressure.

You know, if your radiator is over 8 years old it's probably a good
idea to replace it anyway (only $80).  All of the hoses too.  And the
water pump (only $30).  Cheap insurance.

Mike