Probably running/walking past the smoke alarm.

There's a fine line between too sensitive and not sensitive enough.

If I burn something while cooking I do NOT want the smoke detector to go off (it's loud), and the Wave feature takes 3 or 4 seconds before it registers.

Plus there are a lot of configurations to account for. Some people put the units on the walls while others on the ceiling. The Nest has to guess how far away people are when the walk by and how far people will be when they wave. Putting them on the ceiling with 10+ foot ceilings verses putting one on the wall in a small hallway will require different sensitivities for the Wave feature.

If you have multiple Nest Protects then they will all go off when one detects smoke or carbon monoxide, so the Wave bug won't matter too much.

The movement of smoke can trigger motion sensors. I believe they tried to account for this by also using a heat sensor, but if the smoke from the fire was within the threshold for "body heat" it could falsely assume a person is waving at it.