Here is a diagram I use on my reports. You have a primary and secondary drain. The primary should has the maintenance stub (you should pour some bleach down the stub a couple times a summer.) If the primary does not have the trap installed before the stub you will lose air through the hole. If it has no trap at all and the run to the sink (the most common termination point in newer construction) is short enough, many people notice a hissing sound and air coming through the drain of that vanity. The TREC also says that the trap should have pipe insulation on it.
The secondary line sometimes just goes to the drain of the pain then outside at the soffit. Some times they will have the secondary drain from the evaporator drain into the pan. Sometimes they will plumb it directly to the line going out and have the pan drain tie in with a "T."
If you don't have a trap, you can buy a trap fitting and install one cheaply. You can also buy a pvc cap and place it over the maintenance stub (don't glue it) so that it stops the air.