If the fluid had been lost, providing you don't think the engine got so hot that it boiled it off, it sounds like you have a leak.
Do you have any puddles under the car? Can you see any blueish streaks from a pipe junction or around the rad? Does the pressure bottle (top-up bottle) have deposits that show water loss? Especially check the link from the pressure bottle in and out as these can leak.
If you run the car around gently does it overheat or is it more when thrashing it?
High rev overheats that drop back to normal when you slow down are likely to be the pump. General overheating is likely to be the rad. Water loss from a leak will, of course, give overheating anyway.
As Alfalincs says, after 6 years it is worth swapping the rad anyway - it will be knackered. Buy one from ebay as they are as cheap there as anywhere. There is a 'shop' that specialises in rads which you will easily find.
Why not fit it yourself? It's not that bad. Search on AO for 'how to really change a V6 radiator' by yours truly
If you get a garage to do it they will probably charge you a few hours.