The only way to see standing water by shining a torch down a drain
is if the trap is directly beneath the drain. I have at least 3 drains
where that is not the case. Kitchen sink with garbage disposal, a two
tub utility sink and the bathroom tub where the trap is at the bottom
of the overflow tube, not under the main tub drain.
In addition if a trap was empty, I would suspect that the odor would
be strongest near that drain, not near the stack pipe.
With all of that said, I don't quite understand how an odor could
come from a "stack pipe" inside a dwelling.
"A stack is described as the vertical section of a plumbing drainage
system of soil, waste or vent piping rising through one or more stories
of a building. The stack is the largest vertical pipe into which all the
horizontal pipes either drain or vent."
The stack typically extends through the roof of a building and is open
at the top.
In order to have a "bad smell coming from the stack pipe" I would think
that the stack would have to be open in the apartment in question and
I can't think of a possible reason - in a properly plumbed building - why
a stack would be open in a first floor apartment.
If there was damage to the stack or a bad connection and negative
pressure in the apartment that could cause a odor. IMO any opening
in the stack, whether caused by damage, wear and tear, a leaky
cleanout, whatever, removes the "properly plumbed" status.