Your description makes me think of the smell from the A/C in my car. You say this is in a business, which would likely mean that your bathroom is merely a toilet and sink, right? If so, I wouldn't think that the bathroom would need that much ventilation, compared to a full bath in a home where you're taking steamy showers and such.

One thing I would check for is a plumbing leak. Perhaps some water lines are slowly dripping, causing a build-up of mildew in the walls or under the floor.

My first thought, though, is that the A/C coils in your system are dirty. I was told that the A/C in my car smells bad because the area that it's built in is so small and tight that air cannot circulate well enough to let it dry out and prevent mildew after use. I've smelled the same smell at one of my previous jobs, too. Businesses are about the only things here with A/C, and it gets used maybe a month or two per year. Since A/C is so rare here, I don't know the terminology that well. The coils that are exposed to your air flow (cooling the air) may simply need cleaning / sanitizing.

Thank you. yes, i only have a sink and toilet, and it's only used about 3 times a day. I poured bleach down the overfill hole in the sink, that didn't do anything. I will have a plumber check for a leak.

Now that I've smelled the odor more, it's kind of like a "sour" smell. There is a coffee shop next door to me, and I'm wondering if that has anything to do with the smell.

Most commercial offices will utilize the space above the drop in ceiling as a return plenum. The return grilles simply dump air into that space and then the unit pulls it in. This means that any fart gases etc. sent into the ceiling space will be sent right out the supply diffuser directly to your nose. now your system may be set up different so raise one of your return grilles up and see if there is any return ductwork tied into it. I bet it is just laying there without any duct. Plus any dead birds etc in there will add a stench that will be unbearable. Several years ago they expanded the office space at our sheet metal shop the clowns installing the new bathroom exhaust routed a flex line over to the supply diffuser over the field supervisors desk THAT WAS NASTY boy. If you had to go to his desk to get a assignment you did not stay long lol