Is my septic tank leaking?
Very interesting question.  My first thought is that dropping 6 inches in 3 weeks is not really normal in a sealed system, but not a condition to be alarmed about.  The goal of any inground leaching system is to exit solid free water to a sandy bed to filter and absorb into the grounds. 
It is very difficult to determine if you have a crack or breach in your tank without inspecting it when empty and cleaned. Usually my septic subs will drain the tank, clean the bottom enough to look for a displacement crack or root infiltration. It is rare that we go to that extent unless there is evidence of effluent waste water surfacing on the ground. This is evident by soggy ground and a foul smell. 

If you do have a minor leak towards the bottom of your tank, it really isn't causing any real harm as the waste water is just taking a short cut. As long as you don't have the symptoms of failure, I wouldn't worry to much right now.  Many old systems actually used just a leaky tank without a leach field. This was common on small lots in areas of sandy soils that drained well.

As long as your septic tank is far enough away from a well used for potable water, contamination should not be a problem.  If you do use a well, a water test might be a good idea if the tank is in closer proximity than the leach field.