I found my septic tank! The nasty thing is that it wasn't anywhere close to where I was digging. A friend if mine who works for the sewer department went through the history documents for me and determined that it was in the front yard, way far away from the deck. An old retired building inspector that i'm friends with said that he'd never seen such a thing in a house from the decade mine was built, especially when the plumbing stack is in the back of the house.
I had to wait until this morning to answer this question. We are cleared to build! We found that there was a dry well in the backyard on the same document that shows the location of the septic. I wasn't sure if they would make me find it and prove it's not going to be a sinkhole someday. They aren't going to make me find the dry well but I had to double promise that if I do find it I'll decomission it. Ever onward, by next summer my back yard will be our little paradise.