I have an HP Photosmart C6300. I bought brand new colour cartridges half year ago and have not used the printer since then. Yesterday when I tried to print something, the ink levels were about 20% down.
Does ink evaporate or do I have a leak?
Simply put - Ink is a bloody con!
I just checked the specifications and you have an all-in-one... I had an all-in-one Epson and found the same - I bought 4x full cartridges and mainly used it for scanning, I went to print and it had gone down by about 70%.
I found out that EVERY time I turned it on, it would deposit a tiny amount of ink. If you do head cleaning or any of the other maintenance tasks, it does a lot more.
However, I think it was just the chip on mine - I got a chip resetter...
And was able to use this 3-4 times before it was really empty - I am not sure if you can get something similar for HPs though.
Yes, ink evaporates, mostly through the hole required to let air in when ink is taken out. This is why they are sold sealed airtight.
If this was a perfect world, someone would have added a trivial fix: a rubber pad to close the hole in each cartridge while the printer is not in use. But, alas, it is profitable for printer manufacturers for your ink to evaporate...
P.S. Strictly speaking it's the solvent that evaporates of course; the dye stays behind. If the dye could evaporate too it would leave the paper blank after a while!