My wife had a 55 plate 1.9TDI (PD) that had similar symptoms. There seemed to be no noticeable leak at all, and then while driving the low coolant warning light would come on and there was a huge loss in coolant, but nothing was wet!

After the dealer had the car for a week, they were about to give the car back as they couldnt get it to happen, when it happened as I've described to them. They eventually got a Skoda tech down to investigate. It turned out to be a faulty EGR valve that was occassionally failing and dumping the coolant down the exhaust - hence nothing was wet as it all came out of the exhaust.

This might be worth investigating on yours.

Am going to start with the water pump as I do see some slight wet patches on the under tray and take it from there

My 55 plate Octavia 1.9 TDi is losing coolant, same symptoms as yours; no physical sign of leaks, pressure tested, sniff test and garage cant see any water puddles etc. etc.

 

Just drove to work in it (about 80 miles) warning light came on just as I got to work. Empty!! Had a look and we could see signs of water on the spar just underneath the reservoir and around that area, felt all the pipes and jammed my hand in and under as best I could (topped the water back up and had the engine running) couldn't feel much. Ended up taking the reservoir out and found the rubber hose leading to the engine was moist all around the mouth. Cut the hose back to get some fresh rubber and replaced the spring clip with a jubilee clip. Hopefully that should solve the problem as the rubber was quite stretched and there was just enough tension in the original clip to keep the hose secured. Will let you know as I really can't be doing with replacing the water pump.

 

Only answer I can think of is the hose must stretching at the mouth under the pressure and slowly leaking out fluid.