Overflowing private dumpsters, leaving garbage in the rental car, just leaving it in a heap somewhere, etc. The tell tale dust gives it away. The issue isn't people who stop by the Reno waste processing facility and pay for it to be tossed, it's the people who decide to dump in the city instead of in the desert.
If you have a lot of trash, most economical option is often to go to the public transfer stations or landfills in the Reno area (but that only works if they are open).
Also, there are services on the side of the highway that accept trash for $N per bag. Only give your trash to someone if you can see the dumpster it’s going into and the dumpster is not full. There have been scams where people charged to accept trash and then just left it there to get blown around the desert. Alternatively, you could drive your trash all the way home and let your local utilities handle it. But when I’ve had my cargo trailer piled with leaking garbage bags I’ve wanted to get rid of it ASAP.
1. Burners drop trash with people offering to dispose of it for $5 a bag or so, they end up dumping it somewhere else.
2. Burners drop trash at the major hotels and casinos, who buy dumpsters to attract the burners and then people from the local area toss the dumpsters scavaging for things.
3. The normal trash dumping issues Reno has are, for two weeks a year, blamed on burners instead of the locals. I seriously doubt trash bags full of baby diapers, mail and construction debris (all examples I see on the Reno subreddit every year) are from Burning Man.
There are already legit places in Gerlach, Reno, Lakeview and Ceaderville where you can dump trash and know its going to be disposed properly, but not everyone going is really hip to spotting the trash scams and all that.
Unfortunately all this mixes with the percentage of people going to Burning Man who don't dispose of their trash respectfully and it becomes a large, hard to quantify issue.
Going back to the event itself, I attended Lightning in a Bottle once. I was absolutely disgusted at the end of it. Entire camps quite literally just left, abandoning everything. Brand new equipment and the boxes it was sold in just left for others to deal with. And not just isolated groups either, people had done this absolutely everywhere.