According to a recent Public Health Services Report entitled "FoodNet estimate of the burden of illness caused by nontyphoidal Salmonella infections in the United States", for every confirmed case of Salmonella, there are 38.6 (on average) that out in the public that went undocumented.

After estimating the number of culture-confirmed infections in the United States, we extrapolated, using “multipliers” of surveillance artifacts ...to estimate the total number of Salmonella infections. Using this method, we estimated that there were 38.6 cases of Salmonella infection for each culture-confirmed case. Using a similar method, Chalker and Blaser [2] calculated a multiplier of 39 to estimate the total number of cases of salmonellosis in the United States, including asymptomatic infections. Mead et
al. [5] used a multiplier of 38 that was based on preliminary FoodNet data.

To put this into context of the latest outbreak, Reuters is stating that there are currently 756 confirmed cases of salmonella. If the above report is to be believed, then the true number of cases is over 29,000.