Firstly, I believe you should have written a summery that depicts how far along Harry and Hermione are in their relationship--if there is one. Primarily because the story gleams no insight in this regard, lending to the idea that this is one "hell" of a way for Hermione to enlighten Harry.

Unfortunately this point of hypothetical humor is lost to me as I really don't know what to believe.
Secondly, I can plainly see the potential for great humor with this piece of yarn; however, I found myself utterly confounded at the crux of all good humor; the delivery of the punchline. You wrote:
"I don't know what those are doing in here," she said stiffly, straightening her robes, "must've been a mistake," Hermione finished, finally chancing a look at the room's other occupants.
OK, she is definitely looking at Ron according to what you have written (which is why I believe you should have written: "finally chancing a glance at the room's other occupant"). You continued:
Ron cocked a brow and narrowed his eyes, not believing a word she said, while Harry stayed rooted to the spot, unable to move. Hermione rolled her eyes and sighed, kinky tendrils flying up in exasperation.
She is still looking at Ron when she begins to utter her next words according to what you have written (which means you have to apprise your readers in some written manner of her adjusting gaze upon Harry; unless you negate the necessity of this by changing the word "look," as previously mention, to something that indicates a "brief look." e.g. glance). You continued:
"I didn't think there would be an audience when you found those; you were supposed to be five hundred meters away on a broomstick. Be that as it may, I guessed correctly. Second bed on the right, across from the tacky orange one. I have quite an extensive collection, you know. That was just a preview. If you're not into lace I can show you something else..." and she turned on her heel and glided out of the dormitory, not daring to glance behind her.
The first half of the first sentence in that paragraph makes sense if Hermione is speaking to either Ron or Harry; however, the second half of that sentence is the most puzzling.
Hypothetically, if Hermione had uttered that first half to Ron, she had to of been speaking the second half to Harry; meaning that Harry would be out of the way for Ron to find them. Although that would make no sense as there was no transition mentioned of whom Hermione was speaking to between the halves of the sentence. Of course, it would also make no sense as this is a Harry and Hermione tale.
Therefore, she had to be speaking to Harry; however, that begs a question: why utter the second half of that sentence at all? Did she intend for a house-elf to deliver her knickers to Harry on his broomstick? No, that can't be so. That would not concur with the gist of the rest of that paragraph.
Do you see what I mean by me being confounded?

The first half of the first sentence coincides with the essence of the bulk of the paragraph were the sentence resides, the second does not. What does being "five hundred meters away on a broomstick" have to do with finding the knickers in his room amongst his laundry?
Anyway, I did laugh, only belatedly.

I also wouldn't mind perusing any addendum to this nice little story.

Good job overall.
Also, I usually don't read one-shots but I thought I needed a laugh, so thank you for that.
Rick