I know this problem from my work, not only with LibreCAD, but with proprietary CAD/CAM software too.
In LibreCAD we have no special tool for this. Iirc, there are feature requests addressing this, because this is a problem for LibreCAD's Hatch tool too.
I know cases, when accuracy and rounding errors of the CPU or software cause this.
But mostly, as you already mentioned, multiple iterations and modifications of the drawing cause this.
What I do in LibreCAD to find gaps in a contour, is to use the Polyline tool.
The tool named JOIN in other CAD software is
Create Polyline from Existing Segments in LibreCAD.
The character of a polyline is, that it is stored in vertices. This ensures, that each segment, lines and circular arcs, are exactly connected.
When the polyline tool hits one of these gaps, it stops creation at this point.
If the polyline is only a part of the contour, there are at least two gaps, one in the starting point and on in the ending.
But when the polyline contains the whole pocket circumference, there is only a chance for one gap in the start/end points.
To check this, the List plugin helps, it shows the vertices and the first and last one must be identical for a closed contour.
When the CAM software don't like polylines, you can simply revert this with the Explode tool.
investing less than half an hour into Search function can save hours or days of waiting for a solution