The level seems low?
The loss of wine from the bottle by evaporation or leakage is called Ullage.
Evaporation is a perfectly natural effect. It will happen as cork is a little permeable, in bottles which are aged 10 years or more. It shouldn't happen in a young wine.
Signs of leakage include wine seeping round the cork, protruding cork, and stained labels.
Note that some of these can also be left over from bottling, and we'll be happy to advise if you spot some stains around the cork, so give us a ring!
It can happen in storage by allowing a bottle to become too warm, when the wine expands, forcing the cork out of the neck of the bottle, or causing wine to seep. If the time between such loss and consumption is relatively short, the wine might not to have suffered and be just fine. However, if the wine cools down again, contracting, air can be sucked back into the bottle causing deterioration.
It can also happen with age if the cork dries and shrinks a little.
The best advice is to ensure the wine is stored out of sunlight, in a cool place and with the wine in contact with the cork.