The initial 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 returning to the restaurant from the grease trap. The ones that dry out are often ones that get hardly any 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 over 60 days since the last pumping, getting your grease trap pumped out may be the solution.
Clean floor drain cups. If you have floor drain strainers that catch debris, make sure to clean them out periodically.
Foul odors are bad news for restaurant business. Coming to the source of the problem may be very frustrating because the smells reoccur.
If the odor is coming outside around the grease trap, ensure that the manhole 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 as well as 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.