For a strain of Salmonella to persist in a facility for over a decade is not unusual, and points to the difficulty of eradicating an entrenched environmental contaminate like Salmonella. In a news article, it was stated that the current outbreak began after construction was conducted. "It is thought that the disassembly of partitions and repair of floors in the building at the beginning of 2017, may have led to the accidental release of the bacteria, despite controls put in place to contain the spaces under construction." And this type of event may provide the organism to reemerge from a subterranean location. If they occurred, probably not at a sufficient level to raise concern.
I think it is also important to point out that the company had been doing testing. The facility "conducted more than 16,000 analyzes on finished products, all of which proved to be compliant." This points to the limitations of testing when one is looking for small levels of contamination.
BBC News
http://www.bbc.com/news/world-europe-42908706
Lactalis baby milk scare in France: Salmonella taint 'began in 2005'
1 February 2018
BBC News
http://www.bbc.com/news/world-europe-42908706
Lactalis baby milk scare in France: Salmonella taint 'began in 2005'
1 February 2018