Not sold on only running the tender when I'm home. That'd be the last time I want it to blow up!
Ok, slightly more seriously; would be interested in the Fire Marshall's report when it's done. Have seen batteries blow up before when being charged. Was something we used to have to be very careful about. Even still have a couple small acid stains from it happening while under a hood and bumping a charger's cable. But that was when batteries weren't sealed and with a big charger.
The tenders are such low output devices. AGM batteries are (at least supposed to be) very hard to cook off too. Where's Gary?
Gary, what does training and/or doctrine say on these things? While it is of course terribly unfortunate that some combustible went off in this man's garage what are we really considering here. First reaction is that it's such a rare thing I'm not losing sleep over the possibility.
Not knowing all the particulars about this incident I can say that the older (wet cell) batteries can get hot and overheat when charging. I've seen/had this happen.... at one time there was a disclaimer on chargers to make sure you use them in a ventilated area, away from all other hazardous materials/vapors, and make sure the battery caps were loose or off. With the newer gel (AGM) batteries I haven't heard any problem such as what was described in this incident... this is not to say that something could/would go wrong and create this situation. I've used a battery tender on my bike(s) since shortly after getting my first H-D back in 1999. I leave them on all the time when the bike is parked (I guess this is one time me not riding a lot will be a testimony of how safe {IMO}

) a battery tender can be.
It is possible that "anything" could have gone wrong and the "Battery Tender" could have played a part in this accident, however I'd like to know more about what type of "Battery Tender" was being used, what type of battery it was charging, what type of aerosol cans were in the vicinity, etc, etc... Like you mention the final incident report from the FMO will give more information to the questions I have.

Ride Safe,
Fired00d