About how Zombie threads come back to life

Right now, there is a thread in Factual Questions titled Snapple “Real Fact”, about brain waves and electric trains. In the list of threads there, it is number 18, with the most recent post 16 hours ago. (That would be around 6 PM Thursday, Eastern Daylight Time)

But when I go into it, the last post is from April 2005.

I very nearly posted something there, but then realized that I’d be waking the zombie. But, no, something happened yesterday and it already got woken. What’s going on? My guess is that someone posted yesterday, and then a mod deleted it. Can anyone offer more info? I’m just trying to undetand how the message board works.