Coolant Issues

Just purchased my first A4. Its a 2003 1.8T Quattro with 40k on it. I noticed the other day when I was checking my fluids for the first time since at the dealer that my coolant level was low. I filled it up, checked it a few days later and it was way below the minimum level. I filled up again, checked carefully the next few days for a possible leak (puddles after i park, browsed around the engine, smells of coolant from outside the car) and found none, but the level has still dropped more than normal.

1. Can someone please give me some more things to check? If any?
2. Possible reasons for this?
3. How worried should I be?

Thanks for all your help.