Foul odors are bad news for restaurant business. Getting to the source of the problem can be very frustrating because the smells come and go.
The first step is to make sure no floor drains have dried out. The water barrier in the bend in the floor drain keeps sewer gas from coming back into the restaurant from the grease trap. The ones that dry out are often ones that get very little use, e.g. a floor drain that is under a cabinet. Pour half a gallon of water down all your infrequently used floor drains.
If this does not work, check the last time you had your grease trap pumped. If it has been more than 60 days since the last pumping, getting your grease trap pumped out may be the solution.
Clean out floor drain cups. If you have floor drain strainers that catch debris, be sure to clean them out periodically.
If the odor is coming outside around the grease trap, make sure that the man-hole cover is flush with the rim that it sits in. If this is the source of the odor, try increasing the frequency of your grease trap pumping or using bacterial additives that dose your drain line periodically. These microbes digest the grease and foods solids helping potentially helping you extend your pumping frequency and also reducing the source of the odor.
Finally, try finding caps that sit under the man-hole cover that provide a more robust seal to the grease trap to reduce odors.