Is writing an outline necessary for foreshadowing?

Outlining is not absolutely necessary, but its a useful tool. You can also write yourself notes along the way so you don't forget the loose ends you have to tie up.

For instance, I have one that says "4th abductor --the African--not seen leaving the scene. Where the hell did he go?"

I also have one that says "Boss flew to San Juan with computer. Crutch and Bill are in Nashville. Need them in Wyoming somehow." Would be nice if I could get the African to show up too. Much easier to get all the rotten eggs at once.

Oh, and there's a note about a woman I need to briefly show in this book because she'll be the villain in the next.

I outline, but new stuff and ideas still comes up when you're writing. Just make a note that you need to foreshadow this and find a way to add it when you're revising. Sometimes it easier to go backwards and add necessary foreshadowing.

However, it depends on the type of story - for example, if its a murder mystery where during the investigation you're gradually revealing a series of events that lead up to the murder, and at the end you reveal the twist that identify the culprit(s), then you better have that series of events, twist and all, pretty darn clear in your head. Otherwise, Contriveville, here we come.

Nope! However, you may want to take notes.