I was a partner in several restaurants years ago, and helped contract and supervise the buildouts.
We had a great contractor, a guy we ended up using on all of them. That didn't mean, however, that his people didn't initially screw up a lot of stuff, which then needed to be fixed. Some of it was almost funny, like the time we had the health inspector in for an opening inspection, and the contractor noticed steam rising from the urinals after the health inspector flushed them all to make sure they worked. The contractor and my cousin immediately tried to distract the inspector by pointing out some good feature of that restroom, and he either didn't notice the steam, or just decided not to bust us over it, because we were developing a reputation as guys who tried to do everything right and who were not corner-cutters. Needless to say, the contractor humbly connected the urinals to the proper (cold) line later that day. Or the time his guy put the wrong sprinkler head in the closet/alcove containing the massively-powerful gas water heater, leading to a deluge in the closet, a zillion gallons of gross dark water rolling across the floor of the dishroom and stockroom, and a visit from the fire department (and a red tag) late on opening night, after the thing had been running hard to service a volume of hot water demand that it hadn't previously experienced. The staff initially yelled at me to come in the back because the hot water heater had "burst", but it only required me to take one look at the color of what was rolling across the floor to know that that was sprinkler water, and the fire alarm sounding and flashing was my confirmation that I was right. Well, at least we knew that the sprinklers worked and that all components of the fire alarm did too. The Fire Marshall was nice enough to point out that we had the wrong sprinkler head in there, and the contractor humbly replaced it with the right one on an expedited timetable driven by my very-serious threat to kill him.
All this to say that the most obvious explanation -- that there is no P-trap in that drain -- is far from impossible. Your floor drains are likely going to have been routed to your grease trap, and your grease trap is the nastiest, most revolting concrete box in the universe, producing odors that are so horrific that they will immediately induce vomiting in a pump-truck operator who has "only" had to pump cesspools and septic tanks previously. I would rather smell decomp at a hundred maggot-and-fly infested crime scenes than stand next to the pump truck when the covers are off that thing. It is truly Satan's Cologne.
If there is no P-trap, or they have somehow installed it so that it siphons while your restaurant is operating, that's why there is a smell. You can kill with bleach almost any odor that would be caused by contamination on the parts of the drain you can see right there. So what you are smelling is coming from somewhere else, and is almost certainly sewer gas (or in your case, grease trap gas). I'm guessing there is no P-trap, but if there is, why is the P-trap siphoning? Maybe they installed an S-trap, or it is not properly vented and is on the same line with a device that produces substantial drainage flow (toilets, a dishwasher, whatever), so whatever water goes in the trap is being sucked out. The drippings from what I am guessing is an ice machine or ice bin feeding into that PVC pipe should be enough to keep it from drying out in normal operation, as should the water that your cleaning crew should be sweeping into it every night.
One other possibility is, I suppose, that it isn't the drain at all but rather is some other nastiness nearby. If that PVC pipe isn't an ice machine but is the drain from a beer tap -- watch out! If you don't clean and bleach that line, starting at the trough under the beer taps, at least twice a week, assuming you do a reasonable volume of beer sales, the stench from decaying beer can be overwhelming. Ice machines need to have their cleaning program run at least monthly (even if the manufacturer says every six months), and cleaned with sanitizer everywhere the manufacturer says to sanitize it, or they develop a nasty muck in the cube-formation area and everything that drains from there. In my humble estimation, less than 5 percent of establishments properly clean their ice machines on an appropriate schedule, which leads to bacteria growth, muck development, and customers getting sick but not blaming you because they, unlike the FDA, don't realize that ice is "food". A health inspector who wants to break a restaurant's balls will make a bee-line for the ice machine, and will come up with a critical violation almost every time. And that muck reeks. (The reason every health inspector doesn't check the ice machine thoroughly on every visit is that on many of them, you have to pull off some panels to get to the real nasty stuff, and lazy people don't want to bother, which is the same reason that most people don't clean them properly.)
I'm very curious what you find, ultimately, so please let us know.