Hi,
First of all: Please state your QCAD version?
Goto Menu > Help > About
- Mine as example:Win7 32 bit wininstaller
QCAD Professional
Version: 3.25.2.13 (3.25.2.13)
Internet: QCAD.org
Build Date: Feb 12 2021
Revision: cad2d36
Qt Version: 5.13.2
Architecture: i386
Compiler: Unknown
What you call ‘work on the job’ is the mystery part.
You didn’t provide any clue yet on what actions that might trigger this.
And no, I can’t come up with a method that logs your actions. 
- I know of one such thing that behaves somewhat similar: Null-entities.
After Save/Revert, they pop up at the origin.
But those never resulted in visible lines as they are zero long.
Documented a few methods, filed some bug reports, some got fixed.
So the method to verify if unwanted lines appear with what you did recently:
File Save … File Revert.
You should be able to narrow it down to a specific action in a few trials.
Now follows a lot of guessing … 
File ‘11440’: Revision with remarks ‘11440_Re’
11440_Re.dxf (274 KB)
- Don’t know what came first (1) or (2) ?
But clearly there was something in the smaller rectangle delta (13.25, 13.25) from the top right corner in (1).
It also seems that the indicated spook line in (2) is copied perfectly to (3).
That can only be done on intention as the startpoint in (3) isNot (0,0). - (3) is assumed to be the result of ‘later work’.
I placed an overlay of the outer contours of (2) on (3) so that it makes a little sense.
At all the X mark I then have an offset of 0.187 inch (4.75mm).
It then boils down to 7 spooks from (0,0) to where you trimmed two line segments of the offset.
And none at line-arc trims. 
What method did you use for trimming?
Can you replicate making the offset and trimming line segments resulting in spooks after File Save/Revert?
If so, clearly document how you create the offset and how you trimmed that.
PS: I indicated some other oddities in red.
I think those are not all intentional.
Some may have originated from what is explained here:
Regards,
CVH