The tank should not be empty unless it was pumped out. The purpose of septic tanks is to settle solids to the bottom, allow bacteria to break down waste and paper so only effluent (water) goes into the septic drainfield and then filters down over stone and sand. The outflow pipe should be approx one foot below the top of the tank so if it’s dry it was pumped or has a leak. If the house was but in 2005 and had water service, the tank could very well be full to the level of the outflow pipe. That is what I would expect.

Contact your local health dept. There should be a permit and inspection reports done by the area Sanitarian. Most septic tanks are 1000, 1250, or 1500 gallons. A 100 gallons of water would fill it one tenth or less. If it’s full of water and there is no water service from the house, somebody either filled it up or it was filled by runoff rainwater or it’s cracked and is taking on ground water. Do you know where the ground water table is in that area?