Seriously, this is not a problem.
The brother of my former girlfriend was an anesthesiologist. I
was interested if it is possible to have something for sleeping
(I am seasick) and I got a longer speech about anesthetics.
If you are not an expert, trying to gas someone does either nothing,
alert you immediately or kill you.
Mind you, this were experts:
and it killed more hostages than bullets and it did not
knock out all terrorists.
Robbers do not gas someone, they are relying on brute force
(smashing the door). Thieves are not stupid enough to
risk killing the victim. Buying a gas detector is lost
money. And really, do you expect that criminals
carry along flasks of sleeping gas which are not even
guaranteed to work ?
This does not mean that criminals cannot knock out victims,
but they are either spiking your food with drugs or using
needles, in both cases they need contact to you or to your
food. The chloroform or ether rag from the "damsel in
distress" movies are mostly fiction, too. They may work,
but they are dangerous (killing you) and unpredictable
(suffocation, not working at all). Another option is
If criminals do not use violence, they simply rely on
stealthiness. Once inside, they plunder noiselessly and
if they are afraid that you wake up, they are now in
vicinity if they are really intend to knock you out.
More info in this thread: I also recommend the calculations
how much gas of different sorts are needed to flood a room
on page two.
Royal College of Anaesthetists
Despite the increasing numbers of reports of people being gassed in motor-homes or commercial trucks in France, and the warning put out by the Foreign Office for travellers to be aware of this danger, this College remains of the view that this is a myth.
It is the view of the College that it would not be possible to render someone unconscious by blowing ether, chloroform or any of the currently used volatile anaesthetic agents, through the window of a motor-home without their knowledge, even if they were sleeping at the time. Ether is an extremely pungent agent and a relatively weak anaesthetic by modern standards and has a very irritant affect on the air passages, causing coughing and sometimes vomiting. It takes some time to reach unconsciousness, even if given by direct application to the face on a cloth, and the concentration needed by some sort of spray administered directly into a room would be enormous. The smell hangs around for days and would be obvious to anyone the next day. Even the more powerful modern volatile agents would need to be delivered in tankerloads of carrier gas by a large compressor. Potential agents, such as the one used by the Russians in the Moscow siege are few in number and difficult to obtain. Moreover, these drugs would be too expensive for the average thief to use.
The other important point to remember is that general anaesthetics are potentially very dangerous, which is why they are only administered in the UK by doctors who have undergone many years of postgraduate training in the subject and who remain with the unconscious patient throughout the anaesthetic. Unsupervised patients are likely to die from obstruction of the airway by their tongues falling back. In the Moscow seige approximately 20% of the people died, many probably from airway obstruction directly related to the agent used.
If there was a totally safe, odourless, potent, cheap anaesthetic agent available to thieves for this purpose it is likely the medical profession would know about it and be investigating its use in anaesthetic practice.
ADDITION: The portable gas alarms are not intended for narcotic gases, but should warn the user of toxic fumes endangering your air passages (caused by combustion like a fire with burning plastic e.g. hydrogen cyanide, hydrogen chloride/fluoride), choking gases (carbon dioxide, carbon monoxide) and explosive mixtures (Butane, propane).