|Lau Bannenberg Venture-Lieutenant, Netherlands—Leiden aka Ascalaphus|
I think you should be open with the players about much of the mechanics; this isn't something occult, brushfires is something medievalesque people are somewhat familiar with. Peasants burning fields after harvest and such.
So I'd tell them what actions can put out a fire, and that any squares adjacent to fire could catch on fire too. I wouldn't give them the odds, but would tell them what the general principles are.
This is compounded because almost no scenario I've seen is consistent with how fires work; every time it's something the writer cobbled together. While realistically, fire works more or less the same across the world. I don't think players should be the victim of vague rules.
As for hinting that putting out the fire is a good idea: you could emphasize that the garden appears to contain rare plants.
In general secondary prestige is often tied to doing good if the opportunity rears its head:
- If there's fires, put them out
- If there's prisoners, free them
- If there's slaves, liberate them
- If there's horrible monsters, end them
- If there's something that makes the Society look bad, fix it
etc. etc. ; After a while players should see the general pattern