How come? "The coast is clear"?

The URBAN dic. says;

"The coast is clear" is said when it has been determined that it is now safe to proceed; that there are no obstacles or witnesses to what one is about to do.

And one of the example usages shown as;

"The coast is clear", he thought as he watched his lover's husband drive away from the house.

This is quite understandable for me.

However,the next BLOND JOKE, does not sense to me.

I don't know why "if the coast is clear."!

If anyone could clarify it would be greatly appreciated.

* The phone call...

A married couple were asleep when the phone rang at 2 in the morning.

The wife (a blonde), picked up the phone, listened a moment and said,

"How should I know, that's 200 miles from here!" and hung up.

Curious, the husband said, "Who was that?"

And his lovely wife replies, "I don't any idea who it was.

It was some stupid woman wanting to know "if the coast is clear."

* Another one;

Is The Coast Clear?

The doorbell rings in the middle of the night, and the wife gets up to see who it is. She comes back to bed and the husband says, "Who was that?"

The wife says, "Oh, it was some woman." The husband is freaked out. He says, "Well, uh, what did she want?"

The wife says, "She wanted to know if the coast was clear!" The husband is really nervous now. He says, "Well, what did you tell her?"

The wife says, "I told her the coast is a hundred miles from here; how in the heck do I know if it's clear or not?"

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Thanks