This is a trait of people who have the tendency to outwardly express their every thought or feeling in the moment rather than introspecting. Some people are "fleeting" in the sense that they go from thought to thought or action to action, all in the moment as they are occurring and they express or verbalize this process to various degrees. The main thing is that they are less inclined to introspect first. Doing this makes them feel happy and energized too.
I actually disagree with this.
To me, being unobservant is not about being an Se- or Si-user. Rather, I'd say being unobservant or absentminded is a trait of extreme introversion. (So I suppose it would technically be more for Si-doms than Se-doms).
As an Si-dom, I don't really do what is described in the OP in that I don't tend to lose things and complain about not finding them. It's that I'm just not aware of my surroundings.
Sometimes I'll have conversations that go like this:
Person: Hey did you notice blah blah blah??
Me: No...
Person: Oh well what about what so-and-so was doing??
Me: Uh...no...
Other times it's like I forget what is happening or what I just did. In my chemistry class, I've hooked experiments up backwards and dumped acid behind me because I forgot there was still liquid inside.
Edit: Oh and just now I was looking for a bottle of pop everywhere in the kitchen and I couldn't find it. I went upstairs to do something, came back downstairs, and found it sitting right in the center of the table. D'oh!
For a sensor, I'm really quite a terrible one.