Who last messed with the oil cap under the hood. I suspect they broke it and never told you. **** like this happens all the time. Granted the plastic gets brittle during 16 years of high temp and heat cycling, so it probably snapped off at a weak point when someone put too much force into removing the oil cap.

Sounds paranoid but if I don't do it myself, I always go 1/4 mind down the street, shut down. Pop the hood after a few min, check the dipstick to see it is still full to. Check that the drain plug is tight, and also check to see the oil cap is threaded right and tight.. without that extension tube you'd have oil all over the place under the hood.