First you need to "key" the hole - make the existing concrete wider at the bottom than the top surface. If you choose to use rebar you should drill into the slab horizontally and epoxy in stubs to attach a grid you'll make.
Since you say the rest of the slab has no rebar I would not. But you might think about getting ground up fiberglass to add to the concrete to fill the hole, does roughly the same thing.
When you pour the concrete be sure that the hole and surround is dampened first and there are binders that will help. Maybe start with a gravel layer, then a sand layer (builders sand NOT sandbox sand). Tamping each as you go. Tamp to settle not to compress. Then pour in in wet-ish concrete rough smoothing as you go. Make the final layer just a bit proud. Do your final finish troweling out from the center to the edges. Be warned - getting a finished surface to match the rest of the slab is difficult.


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As it's up against a wall, so unlikely any heavy vehicle will be on it, and you "may" sometime in the future need to get back at those pipes, I would think about getting a steel plate to cover it.
Either bolt it down to the concrete or have a metal shop make you a frame and a square manhole cover to fit to size so everything is almost flush.

 

Put the dirt back in, tamping it down, bring it up to the bottom of the existing slab. For a 2x2x6” hole you will need 4 bags of 80# sakrete. Mix according to the directions … easy on the water…. pour in the hole and smooth it. Done.

 

FYI sand has limited compaction. Use sharp cut gravel instead.

 

As @huesmann said - you want sharp gravel 1/2"-3/4" as a base and compacted as best you can. However, pretty much no matter what, the soil will compact more in the years to come - therefore the new concrete "plug" will shift and you'll have trip hazards where one surface is higher than another.

To help prevent and shifting you'll want to "pin" the new concrete to the old concrete. It doesn't look like this spot is designed to hold a heavy car, etc. So, what you could do it get some 3/8" rebar and cut about nine 12-16" lengths. After filling and compacting, and using a hammer drill, make three holes per side into the (near) center of the old concrete. Make sure you put the holes going right to left at one height and the holes going forward and back about 1/2" lower or higher so that the rebar will not be hitting each other.

Set each rebar section into the holes using epoxy as the fixative (comes in a funny, big syringe and will self-mix as you inject it into the holes. Tap the rebar into the 1/2 filled holes (some epoxy may squeeze out). When each section of rebar overlaps with other sections (perpendicular, as well as the rebar from the opposite sides, use wire, or even zip ties to hold them together and in position.

When the epoxy is set, pour your cement with confidence that it will be strong and won't shift.

Concrete is an inexpensive and easy product to work with if you take your time and do it right the first time.

If you look at the drawing - you'll notice (on purpose) that the rebar doesn't have to be pretty (but you could lose style points!).

I'm sure flat-work pros would have greater pointers for you - but, this method would certainly be pretty easy and permanent plus remove the need to adding fiber to the concrete mix.

 

Wow! Thanks for the advice. You’re all right that no car will be parked on this concrete, because its on the side and the garage door won’t permit it. I’ll update when I do the job

 

If you are not sure you can compact it good enough, cheat.
Drive 4 2x4 stakes down into the mud and cut them off at the height that will be the bottom of the concrete.
And then back fill.

 

Ok. I just remember doing new bank branches, and the civil engineer required all organic and deleterious material to be removed from under the building.

 

They, they generally want that, but it's most critical when you have a slab or spread footing. Not as critical for piles, but for piles you're going to have little/no organic/deleterious material to begin with at the depths piles will go.

 

To compact the a small area of soil, before you put in the rebar, you could rent a tamper it would fit in the hole easier, Or borrow, if you don't have one a heavy sledge hammer. With the handle up use the head by raising it straight up by the handle and driving it down on the soil until it doesn't dent the soil. It will compact most everything but sand or mucky soil, and your arms will look like Popeye when your done.