A battery is not itself a source of ignition. Batteries do not sit there emitting sparks or arcs, if that is why you are concerned, so a battery actually presents no greater peril being in the trunk (or under it) than say......a spare tire or a jack.
As to the hydrogen gas (don't forget the oxygen too) venting from the battery:
I have not seen how the battery containment enclosure is configured on any of the cars that place the battery in a "trunk accessible" location..........so I can't say whether (or not) the battery containment provision is actually sealed off from the rest of the trunk and provided with its own "vent to outside air". I would surely hope that the auto maker provided some type of ventilation scheme to evacuate any gases that might vent out of a battery.
And that right there brings into the picture another aspect of (modern) batteries. I would really hope (for a number of reasons) that the batteries in these cars are sealed, maintenance free type.
Sealed lead-acid batteries (of either the "gel cell" or the "absorbed glass mat" type) DO NOT vent hydrogen and oxygen out of the battery case in the same fashion as "old school" lead-acid batteries because these new tech batteries have a valve in the vent port to maintain a very slight pressure head inside the battery. These Valve Regulated Lead-Acid (VRLA) batteries actually vent very little hydrogen and oxygen out of the battery case because the 2 PSI pressure head permits the 2 gases to "reunite" and turn back into water. This is also the basis for VRLA batteries to be called "maintenance free". By allowing the 2 gases to turn back into water, you never need to add any make up water to the battery. And because no venting takes place (under normal circumstances) no acid droplets are displaced from the battery to corrode the cables/terminals and other metal brackets associated with a battery mount. Another reason no acid escapes from VRLA batteries is because the electrolyte is not held in the battery as a "free liquid".
Gel cell batteries electrolyte is in the form of a thick gel (due to additives put into the liquid acid).
Absorbed glass mat batteries have all of their electrolyte held in an absorbent mat comprised of extremely fine glass fibers.
Because the electrolyte of both types is not free to slosh around inside the battery case.....electrolyte does not get tossed against the vent port where it can escape if the valve were to open.
And to add to the concerns (or absence of) about flammables or explosives in the trunk........what about all those stop light, tail light, turn signal, and back up light bulbs and all the associated wiring right there in the trunk?

"These Valve Regulated Lead-Acid (VRLA) batteries actually vent very little hydrogen and oxygen out of the battery case because the 2 PSI pressure head permits the 2 gases to "reunite" and turn back into water."

Doesn't sound like much pressure, but on a standard size car battery (say 6.5"Wx12"Lx8.5"H) that's up to 200-lbs of force pushing out on each of the long sides. Those must be pretty stout cases....not saying they're not though...

*****have not seen how the battery containment enclosure is configured on any of the cars that place the battery in a "trunk accessible" location..******

On the cars I looked at, the battery is in the same well as the "spare" tire. A piece of carpeted composite similar to Masonite or equivalent covers the well. Same as it has been since the 1970's

Biggest danger of gas cans in the trunk is :
filling the cans while they are in the trunk, or in yer pickup, or on a plastic bed liner---they can ignite the gas fumes from interactions with the plastic or metal can and the bed liner, or trunk floor covering. Always take the can out of the trunk, or truck bed, and set it on the ground to fill it. If the fumes are allowed to gather in the trunk, and somebody causes a spark in the car--BOOM, and away ya go!