Nope, it's not missing media as in the Premiere timeline. The stripes can appear on adjustment layers as well as footage.
I'm pretty sure that, in AE, you are prevented from extending the layer if there's no media 'beneath' it.
See this page for an annotated diagram and this explanation:
"You can extend many kinds of layers for any duration, extending their In points and Out points out past their original times. This capability applies to time-remapped layers, shape layers, layers based on still-image footage items, camera layers, light layers, and text layers. If you extend a layer back in time so that the layer extends into negative layer time (past layer time zero), a series of hash marks on the bottom of the layer bar indicates the portions of the layer that are in negative layer time. This indication is useful if you’ve applied effects to the layer—such as Particle Playground or Shatter—that use layer time to calculate their results. "
Thanks Todd for putting me out of my misery at last.
I've ploughed through the manuals since AE5, read several CIAB publications, Chris Meyer's and Angie Taylor's books and never found an answer.
Until today. Wow!