Your grease trap is either under your sink in a metal box, in your kitchen floor under a metal sheet, or outside your restaurant under 1, 2, or 3 man-hole covers. This device is designed to catch grease and food particles that go down the the floor drains or sink drains in your commercial kitchen. So it is somewhere between where the water goes down the drain and the sewer main. The little ones inside a restaurant are usually 20-60 gallons.
So if you have ruled out that you do not have an indoor grease trap or grease interceptor (these are interchangeable terms) you need to start looking outside your restaurant to find your grease trap. It will probably be a cluster of 2 man-hole covers or it can just be one or as many as three. These outdoor grease traps are usually 1000 gal or 1500 gal.
I have a bunch of man-hole lids around my restaurant, so how do I know which one is where my grease trap is?
The grease trap will be between the kitchen and the sewer main. If the kitchen is at the back of the building and the sewer main is behind the building, the grease trap will be right behind the restaurant. If the kitchen is at the back of a building but the sewer main is in front of the building, the grease trap will probably be in the parking lot to the side of the building or perhaps in the grass in front of the building. At fast food restaurants, grease traps are often in the drive through area