Just came up with an opener, want to know your opinion:

Background: At Uni my umbrella was stolen.

When the target crosses your way:

YOU: "Excuse me, hi, I just wanted to know: Did you steal my umbrella?"
GIRL: "What, no, why, no. Why do you think?" (Confused)
YOU: " I don't know, but somebody stole my umbrella and I hoped it might be you... hi my name is XX..."

Then just follow up and chat...

hahaha nice im gonna try that

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Never mistake activity for achievement.

Not good bro. Not good.

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn

It could work. It's pretty funny and it's direct enough for her to know that you're not trying to dodge the reason that you're there.
I don't like the "hi my name is..." at the end. Instead if she's given the opportunity to respond and then build rapport from there, that would work. Anywayz, nothing proves it better than a field test ^_^

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Never mistake activity for achievement.

The problem with it is that she make take it literal. Unless you are really smiling and laughing, but then it might look like you aren't taking yourself seriously, which will cause her to get a weird vibe. Remember, the moment you open a women, she's automatically on the defense. She's thinking "What is this guy trying to sell me? Is he going to try to kidnap me?" etc.

If it's not rainy out then the question will be obviously absurd, and will likely get an eye roll. If it is rainy out, then she's going to think you are actually looking for your umbrella and might be put off by the accusation and just try to keep walking past since she doesn't want to get caught up in whatever drama this strange guy is trying to bring her into.

I'm over analyzing a bit, but I'm just saying that compared to other openers, this one is a little on the silly side and might not work as well.

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn

Totally understand you puaninja.
I'll try it out, but for sure one has to sell it with a grin and perhaps already have said "hello" to her once before your encounter, so that she doesn't shy away.