Gas fumes

Hi, I had an unusual problem with my 240 wagon the other day. I started to smell gas fumes inside the car while driving. There were no apparent leaks anywhere that I could tell, so I was going to check the o-ring seal at the in-tank pump, thinking maybe it was bad or something. Well, before I got to that, I stopped to fuel up and when I went to take the gas cap off, it just came right off, clearly it had not been on right. I guess I must have been distracted when replacing it. So it occurred to me that this could be the problem, and apparently it was, because after closing the cap(clicking it a couple times as I usually do), and then driving off for a few miles, no more fumes. I guess this is a closed, sealed fuel system in order to work properly. A friend of mine said he had heard that, on newer cars, if you don't close the gas cap tight it could cause the service engine light to come on. I had never heard of that before.