Proably someone will come along with more experience than me, but here's what I know.

1. SOmetimes you can see the tank when there's a light snowfall. The snow will melt over the tank. Not a reliable method, and not available to you for a while yet.

If the ground is relatively soft (debatable if you have clay, but if you've had rain lately, this may work) take a 6' or so length of rebar. Maybe sharpening the end would help. Just keep driving it into the ground in a grid pattern until you strike concrete. Go into the basement, locate where the pipe exits the house. Figure out where this is outside. Go about 4 - 6' away from the house in a straight line (depending on the size of your lot, how much space did they have to work with. Start driving the rod into the ground. Pray that you don't have an old steel tank. In that case, wear life jacket.

2. I think ours is down about 2 - 3 '. Interestingly, you could see where the lines ran during the drought this summer. There was one kind of weed that grew in lines over the runs

3. Be very careful what you put down the toilets and sinks. If this is your first septic system, well, the old saw goes that if it didn't go into your mouth first, it shouldn't go down the toilet. (Toilet paper notwithstanding.)
No grease, such as when you roast meat, and have that greasy ring in the pan.
Might be worthwhile getting the tank pumped. Might give you a month or two's grace.
4. There are some systems where the effluent is stirred and pumped. Sorry, I don't know much more.

I would be surprised if the municipality allowed you to put a new bed into the old location, especially clay. If you are limited by space, my guess would be that you'll hve to excavate and bring in sand and gravel for the new system.

We had a similar problem locating our "D" box for a title 5 certification. A local neighbor/farmer found it for us by using divining rods (I kid you not) and a backhoe. Ours was off plan as well, buried in the side of a hill 10 feet down.

Ask around town for any locals that might remember or have any knowlege of where yours might be. Hope things work out.

Sheila

You never know if you have the ability to "divine" or "witch" something until you try it. Take metal hangers, cut off the hook and twisted top, straighten them out, then bend a four inch section at a 90 degree angle. Put this section in your hand, thumbs up and point the long ends together in front of you. Start walking. When you cross over a water line, telephone line, etc. the hanger ends will swing straight out to your sides. If you keep going, they will point behind you. I kid you not. You either have it or you don't so why not see for yourself. It's fun.

Sarah

Just had to add a bit to this--our local rural electric trucks were here, and needed to locate a underground electric line before they drilled for a new anchor. one of them bent copper wires and found the buried line, said he did it all the time! thats confidence--when you're going to drill for a hot line!!when he found out i could do it also, he had me check--same spot! then, he gave me his wires--said copper were the best.

I've had luck witching for water as well. I was a bit skeptical when I took the branches, but those little buggers bent in the middle and pointed south right now.....stood the hairs up on my arms. The more I pointed them north, the more they bent south until they were practically right angles. Something to it.

We find the box easily in summer as well, because the grass dies in the shape of a square if there isn't enough rainfall.

Witching - or dowsing - does seem to work. I had a clogged sewer pipe, called a plumber to clean it out, and pointed to where I thought it should be buried. He pulled out a single bent wire, walked around, and started digging at least eight feet away from where I'd pointed. He was exactly right! I tried it later with galvanized electric fence wire and found the method described above very easy, but the single wire method he used was much more difficult.