You don't smell the gas if you have been sitting in the room from the beginning, only if you walk into a room after it has been leaking for some time. Happend to me once, I was sitting in the kitchen with my roommate and started to feel a headache. But didn't feel the smell. We were both awake and talking. After some time a neighbour from the flat above us knocked on the door because she could already smell it when she walked trough the corridor and saved us (important, you shouldn't ring a bell in such situation because it could start an explosion). We have been sitting by a stove leaking gas probably for more than an hour.

EDIT: Thanks for the award! The note about the bell seems to interest many people, so I'd like to add, you shouldn't do anything that might cause a spark of electricity, like switching on/off a light or any electronic device or plugging anything out or in electrical outlet. Don't touch anything with electricity and of course don't light a match.