You're Not Alone. And It's Not Just Toyota. Your Experience Is Repeated Everyday By Owners Of All Sorts Of Vehicles, All Across The Country.
Cigroller Has Given You Some Good Advice.
If you don't want to be dependent on the dealer who charges $110 to read your codes or inconvenienced by going to an auto parts store to get it done for free, next time consider purchasing a simple, inexpensive code reader. For example, an Actron Pocket Scan (about $50 at Sears or other brands available at auto parts stores) can read codes and turn off your light for you. It comes with a book that translates the codes (100s of codes that will usually need further explanation).
Link to some codes translated:
http://www.obd-codes.com/trouble_codes/ (Open this and scroll down the page.)
Also, as Cigroller said, "Write down the exact codes (they look like "P1234") and post them here." Keep in mind that the codes are only clues to which system in the vehicle has detected something out of the ordinary and that usually requires further diagnosis. In the case of a "major EVAP leak," the first thing that should be tried is tightening the gas cap and then replacing the cap if that doesn't work. Most issues are not as simple as a loose gas cap.
I usually check for codes, write them down, turn off the light, look up the code and do some reserch, while waiting to see if or when the "Check Engine" light and code(s) return in the near future. Sometimes you'll then get additional codes that help zero in on the fault. Sometimes the light and codes never come back, as could be the case with a loose gas cap.
P.S. $110 probably isn't too expensive if they actually checked and did further diagnoses. Some shops would charge $70 to $120 to read the codes and additional labor to check it.
Your car probably had a code PO455. See what "possible solutions" you could have tried had you read the codes at the store or home.