Sealed lead acid batteries can have a design life of anywhere from 3 - 5 years all the way up to 12+ years depending on the manufacturing process of the battery. There are many factors that affect the service life of the battery including temperature, amount of charge and recharge (chemical cycling), humidity
Unless you logged (tracked) the mileage at the time the vehicle was parked, there is no real method to conclude that anyone has started or driven it, during the time you had it parked.
The primary reason for the relatively short cycle life of a lead acid battery is depletion of the active material. According to the 2010 BCI Failure Modes Study, plate/grid-related breakdown has increased from 30 percent 5 years ago to 39 percent today. The report does not provide reasons for the larger wear and tear other than to assume that higher demands on the starter battery in modern cars induce added stress. The organization conducts a study every 5 years to determine the failure modes of batteries that have been removed from service. (BCI stands for Battery Council International.)
While the depletion of the active material is well understood and can be calculated, a lead acid battery suffers from other infirmities long before plate- and grid-deterioration sound the death knell. These conditions are found under: Corrosion, Shedding and internal Short, Sulfation and How to Prevent it, and Water Loss, Acid Stratification and Surface Charge. Most of these can be reduced by proper handling.