Tap into drain line by basement main drain? Or where?

Just bought a ~1979 colonial in NoVA that has a full, finished basement. I want to add a basement kitchenette (sink and dishwasher). I've done a fair amount of plumbing a DIY'er over the past decade, but I'm having trouble figuring out how to get started on how/where to tap in the the drain line.

The stack is some 18 feet away from the planned kitchenette centered along the back of the house. There's a floor drain is nearly centered front to back, side to side, and is about 8 feet away from where I want the sink to be. I'm guessing (!) that the waste flows from the stack, past the floor drain, to the front of the house where, presumably, the sewer is.

So how do I figure out where to tap in?

sewer

The one thing you do NOT do is guess where it is, unless you want to end up breaking the entire floor. Call a pipe locating company and have them tell you WHERE the sewer is, then you can decide where to make your connection, or better yet, where to have a plumber make the connection and install the drain line properly.

Ok, so I hired a plumber to scope and locate the line.

First a very general question. For a 1979 house in Virginia built as part of development, can I assume that I'll find a bed of gravel before I find the drain pipe, or is it possible that the pipe is laid right in the dirt?

Here's where it gets complicated. The line goes under my stairs. The floor is finished in ceramic tile, and I wasn't left any spare tiles, so I'm trying to stay in the footprint of the stairs.

There's only about a 3' x 3' square of floor where there's practical access under the stairs. I've started out with a 18" x 18" hole through the concrete (4" slab). After that there was about 4" of gravel before hitting dirt.

Immediately below the gravel I encountered the corner of a stack of 8x8x16 CMU's at least three ranks high that are mortared together. They don't seem to serve any structural purpose since a) the slab doesn't thicken there, b) the floor doesn't support any loads in the immediate area (I know where my columns are), and c) the CMU's are separated from the slab by a layer of gravel. They seem to be of the same basic composition and texture of the block used to be build my basement walls, but it's hard to say.

The excavation is very slow going, and I'd like to try to figure out if I can rule out why the CMU's are there before going much farther. I've developed three theories for the CMU's that I'd like to run by you all:

1. They're debris left from building the house.
--If so, why are they mortared together? I don't think that this is likely, but it is possible.

2. It's part of an old structure.
--If so, then would it be safe to assume that the drain would not travel beneath them?

3. They're part of a pier erected by the builder to help set the level for grading.
--The house is built into a hill (first floor just above grade, walkout basement in back). I favor this one because the top of the CMU's is level with the top of the dirt. Under this theory, though, is it possible that the drain could go underneath the CMU's?

quote; I assume

You know what happens when you "assume" don't you. I would NOT have hired a plumber to scope the line. I would have hired a pipe locator company. (I AM a plumber but that is how I always do it) It is very UNLIKELY that the pipe is buried in anything other than the soil that was removed to make the trench. No way to tell what, or why, the CMU pier is there, other than to say that contractor's seldom do "busy work", so it must have had a function. It would be real serendipity for an existing pier to wind up EXACTLY the right height for you basement floor. Whether the drain would go under it depends on WHO was there first, the plumber or the mason.

After searching out that CMU stands for concrete masonry unit, I did this search:
https://www.google.com/#q=slab+"builders+pier"