Can gas markers be set through walls?

[ATOEtalGeek]

You should be applying the Line of Sight rule. The Hero wouldn't be able to "target" (using the term loosely) a space that he cannot see.

As long as the Hero is standing next to a wall he can see through it. If he is standing in an inside corner he can see through both walls, including the space diagonal to the outside corner. If a Hero is standing on a space diagonal to an outside corner he cannot see into the inside corner.

Note that you can throw the gas on the run -so while the Hero is moving. This could allow him to land it just about anywhere he needs to (unless you roll like me)