I don't think that is actually a quest you can get a reward for, but a Tank CAN spawn on the map (it isn't always there).

Try looking for a place that doesn't have an easily visible poster for the tank, where someone didn't want it to be seen. You might need some spend some exp on skills, as well as certain items to get to the tank though.

There is not monetary reward for finding the tank, but you do get a tank so...

There is a building in "Town" that has no entrance. Drop in from the roof after some parkour around the floating house. It has a chance to spawn in there. Once you get in that building, there is a false wall you can break to get it out.