Nice surprise -- I didn't see it coming.
I don't really understand the short-short form. It's not my thing, you know? But for such a very short text to be effective, it must include a twist. You nailed it.
Some of your language is a bit stilted, and some just misused. A basic round of copy-editing will clear that up for you.
- "...or had anything I could compare it to" is ungrammatical.
- "My eyes probed the darkness ahead of me, waiting ever so silently." Eyes are always silent. They don't make any noise. Is this what you meant to say?
- "Slowly, my eyes became used to the darkness that surrounded me..." I got the impression you had been in the tunnel quite a while, lying in wait. Why are your eyes only now adjusting to the darkness? Did you just get to the tunnel? I'm confused.
- "...my body began to tense up as the sound grew closer." Pet peeve. Your body tensed up. Or maybe it didn't. But what is it doing when it begins to tense up? I can't really picture that.
- "...my eyes began to dart to and fro, watching and waiting." Again, I think your eyes darted and you watched and waited. Eyes can't really wait, can they?
- "Suddenly, a shape broke the uniform darkness." If the darkness is really uniform, how is it that you saw the tunnel's twists and turns a paragraph or two earlier? I think you mean to say "broke out of the darkness at the end of the tunnel" or something like that.
- "...having no idea that I waited for it just down the tunnel." What does this mean? Oh wait, I get it. This is a general problem with predicates. I'll address it at the end of the post.
- "...was all that could be heard." This is a little stilted. Why not say "was all I could hear"? If it's all you can hear, then you should say so. If, for some reason, you can't hear it, or it's not all that you can hear, then a story told from your POV shouldn't include the detail.
Okay, the predicates thing. Fair warning: There's a bit of grammatical jargon ahead.
Let's consider this sentence:
My eyes probed the darkness ahead of me, waiting ever so silently.
You establish the subject of the sentence in the conventional way, in the first half. The subject is "My eyes."
Then you add a verb and a direct object: "probed" + "the darkness." So far so good. Subject, verb, direct object...you've got a complete sentence there. The prepositional phrase is well-placed, a fine (but optional) decoration. You've hit the comma with no problems: "My eyes probed the darkness ahead of me."
Then you hit the comma. The second half of the sentence, the predicate, isn't entirely independent of the first. It relies upon the same subject, in this case, "My eyes." So what you have written is, "My eyes...[were] waiting ever so silently."
I think what you meant was this: "My eyes probed the darkness ahead of me as I waited ever so silently." Or something like that.
Yes, I'm harping on a very simple example and an obvious one. But this is a trap that even very experienced writers fall into all the time. When a predicate has an implicit subject, that subject has to come from the first half of the sentence. You can't jump midstream from "My eyes" to "I" without making the change explicit.
That said, it's a very minor error, and most readers won't even notice it. I wouldn't worry about it. I'm a nit-picker by nature and, occasionally, by profession.
Anyway, you did the hard part. You managed, in very few words, to set a scene and deliver a surprise ending.
Well done!