The Royal College of Anaesthetists issued the following statement on 14th July 2014…
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.