There are numerous of stuff they can do... perhaps it is just something similar as has been said before. They joined, but forgot posting. Such things are very common. I, as example, have a hard time getting active on a new forum and usually become quite inactive in the first period - and if it isn't at that moment, then a bit later.
Lately there has been a lot of graphic-stuff, which might attract some people as well who like to see it. There can be all kind of stuff.
But if a member hasn't been active for a long period of time, his account could also we be disabled/removed; there is nothing to worry about.