Let me start by first congratulating Daniel Cohen of Environmental Systems Design (http://www.esdesign.com) for being first to provide the correct answer to the quiz. As a reward, Daniel, you get to have your name set to an Arial 18 bold, italicized, and underlined, bright pink font and a quick plug. Doesn't get much better than that.
(Honorable mention goes to Joel Osburn for coming in a close second). Keep those comments coming in the future.
(Hmm... this is how I should handle all future support cases. Post the question on the blog, wait for answer from reader, forward to customer. Done and done. It's brilliant!) Sorry...I digress...
Interestingly, the third comment posted by Gabe is an indication that many Revit MEP users are unaware of this issue, which is why I wanted to provide this information. You'll notice that Gabe only discovered the issue after changing the connector on an air terminal from Supply Air system type to Other Air (see below)
In fact, most users may never encounter this problem as long as they are using the provided mechanical families, because generally speaking none of the connectors are set to the Other Air system type.
Usually this issue becomes apparent when someone creates a custom family and sets the connector to Other Air. The rationale for this is that they want to create a mechanical system that does not fall under the three primary types (Supply, Return or Exhaust). For instance, they may want to create an "Outdoor Air" system, and assume that the Other Air type is appropriate to use. But as Daniel pointed out, there are no corresponding rise/drop symbols associated with this system type, and therefore no way to display the duct contours for this system.
Here's another indication that this system type was not meant to be used. See the highlighted air terminal to the right? I did what Gabe did and set the connector in the family to Other Air. Note that when I reload the family and highlight the air terminal, there is no tool on the ribbon to create an "Other Air" system. You can right-click on the connector, and click Create Other Air System, but again, this was not meant to be used.
Why is this system type available then? Well, without getting too much into details, this system type was included with a different workflow in mind, that of potentially connecting two different system types together. Suffice it to say, it is not currently meant to be used to create separate "Other Air" mechanical systems.
In summary, if you encounter a Revit MEP model where ducts are clearly shown in 3D or section, but do not appear in plan, before you go through all of the visibility graphics settings, view range settings, and overrides, you may want to verify that the ducts are not connected to an Other Air system.
Thanks to all who took time to comment.