Yesterday, after looking at about a million of 'em online and seeing nothing but good reviews, I found my first mini; a red '06 convertible with 51K miles from a local private seller/young family. As I hit the highway grinning, I naturally had to stick my foot into the RPMs. Looking down, I noticed the "Service Engine Soon" lite was on. I called the seller and asked him about it; and he said probably the fuel cap was loose (and it was--slightly). The owner's manual says the condition "may" correct itself in a few days. I'm a little freaked out so I came here for advice. Is it possible I merely knocked loose some carbon deposits and stymied one of the emission sensors? I have never used a code-reader. Perhaps I should buy one and keep it in the glove-box?
Thank you in advance for any insights...!

 

Go to an auto parts store and have them check the codes. They might not be allowed to reset the codes, but you can push the button on the control if you ask. Come back with the code and someone here will tell you what it means.

 

As Nabeshin said, Autozone and Advance Auto stores will read (and interpret) the codes.
While company policy (I worked for both), and in some areas, the law, forbids them clearing the codes, sometimes they'll look the other way. Another option is Autone will rent the scanner for free (deposit), thus allowing you to do it yourself. They do this, obviously, in the hopes that you'll buy the parts needed to correct it from them.
If the code is a loose gas cap, the ECM will clear itself on a couple of start/stop cycles.
I doubt very much you "knocked loose some carbon deposits", but you might wanna run a can of Shell Techron or Seafoam through the car with your next fill-up.
Oh yeah, welcome to NAM! You're in for a lot of fun.

 

Thanks for the tips. It's great to be here.