What is the warning that you're almost done called?

Consider the following items:

• the warning track around a baseball outfield
• the red stripe near the end of the a roll of receipt paper
• the margin bell on a typewriter
• the rumble strips when a highway becomes a street

They all seem to be of a class: things that give you a warning that something is about to change, that you're almost done. I'm looking for an expression or word for this.

(I was thinking "canary in the coal mine", but that is for emergencies, disasters, that kind of thing -- I want something for the routine changes of life.)

You might be interested in words like cue, sign, notice, signal. They seem imply less danger than warning or alert.

The word warning you used in your title has the following meanings:

1. An intimation, threat, or sign of impending danger or evil.
2. a. Advice to beware. b. Counsel to desist from a specified undesirable course of action.
3. A cautionary or deterrent example.
4. Something, such as a signal, that warns.

I think that the second definition or the fourth capture your contextual meanings.