Using clip markers to drive layer opacity

Hi folks, I need a little help with an expression. Your input would be most appreciated.

I'd like to have a solid layer in my comp which has 0% opacity most of the time, but has 100% opacity only on those frames where there is a clip marker on a specified footage layer, AND the frames where the footage layer has a clip marker on an adjacent frame.

That last part may be confusing, but the practical upshot is that the layer is transparent until a clip marker comes along, whereupon it goes opaque for the surrounding three frames.

Thanks again!
Ari