fire damage in water heater area

[mtpalms]

We got our '71 Safari home yesterday and started ferreting through it in the way new owners will with a new toy. Before we bought it, we checked out all the appliances in the usual manner, through the access doors, and all looked A-okay.
At home, we started opening doors, pulling out bins, and peeking under mattresses and cushions to find all the hidey holes, spare change, etc. To my horror, when I lifted the streetside twin mattress at the bathroom end, the was a huge burned area in the corner. The mattress didn't show any evidence of scorching, nor were there any other traces of recent fire damage that we could see. It appears that there had been a fire in the water heater compartment at a point in time.
We suspect that either A) someone was doing some maintenance, turned on the gas, then lit a match, and created the FWUMP! effect. Or B) left the gas and the water heater on while tooling down the road, causing a flare up, which extinguished itself by oxygen deprivation.
Anybody more knowlegeable than us have any idea what might have caused this, and what to look for as far as damage that may compromise the heater's safety?
This probably happened long ago, as there are no smokey smells, or soot evident anywhere, just a burned area in the plywood.