Leaving my own personal experiences of travelling abroad out of the argument, I take the information I can find from those who actually know about such things, their characteristics, effects and likelyhood of being used.
I think the Royal College Of Anaethetists "may " just have more knowledge than your brother in law and I quote from them
"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 or 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
I also quote from TruckPol
"
Wednesday 11 August 2010 11:08
Truckpol says gas attacks on sleeping drivers remain a myth, despite claims the problem is worsening in some European countries.
DC Alan Soames, head of the crime intelligence unit, says there has never been any evidence to suggest drivers have been gassed so a theft can take place or to enable
stowaways to sneak into a trailer.
"There has been no medical proof so far. [European Law Enforcement Agency] Europol has done some research and were of the same opinion that they didn't believe it's actually happening."
Now,,,,,,,,,,,surely they ALL cannot be wrong???????????