Link: I was officiating a game then. I thought I was coming down with the flu in the worst way.
Hockey arenas are notorious for CO... when it happens, it is often because something wasn't quite tuned up right on the engine of the gas powered surfacing machine. This is not all that uncommon.
All of these types of cases in conjunction withe the one I mentioned in my previous post about houses is all the more reason to always have a portable CO detector on oneself. If you send your kids over to your neighbour's house, how do you know that they don't have a CO problem? Two of the engineers that work for me were doing some testing in a steel plant just two weeks ago and somebody had inadvertently left a test port open on a steel furnace exhaust duct. Flue gas was being pumped out of it and fortunately, there were CO alarms nearby that were triggered. It can happen so easily and as already mentioned here, sometimes the person in the vicinity never knows anything other than she/he seems to inexplicable have come down with flu symptoms. One very big danger is the cockpit of small aircraft... any leakage in the exhaust system will find its way into the cabin. All these are reasons why you must have a portable CO detector on you.