I've brought this up before, but I have slightly more information now. I've noticed since I installed Lion many months ago that periodically, an app that I did not launch will run on my system for about half a second. It appears in the dock for about that long, then goes away. I've never been fast enough to hover the mouse to see the name of the app. The dock icon looks a lot like this:
except that I'm not sure that the arrow looks exactly like that. My memory suggests that it's more of a 2-D arrow, and now that I think of it, I'm not sure whether it points into or out of the box. I don't install a lot of weird apps, but I do sometimes write PHP applications that run on a local server that I create with MAMP, so it's possible that this is related to some PHP package I installed, but I don't remember installing one since I installed Lion.
Does anyone have any idea what this app could be, and in a perfect world, how I can tell my Mac to keep track of which apps have run so I can discover what this app is?