SOLVED. MG/ YA fiction, read in ~1999. Tween-ish girl in a city (NYC?) sees 'HELP' signs in a window across the way and becomes convinced that a child has been kidnapped. Spoilers in post. [s]

[Liralen]

Plot:
Looking for a middle grade or YA novel about a girl living in a city—possibly New York. She may have moved to the city recently, as she doesn't have friends in the area, and she's lonely. She's probably in middle school, and she has an older sister, who is probably in high school. The sister isn't around much. I think—might be wrong—that it's summer, and the main character is waiting for school to start up. From her window, she can see another apartment—a story or so above her—and that someone there is hanging signs that say things like HELP or SOS. The main character becomes convinced that someone across the way is being held hostage or otherwise kidnapped or abused, and she sets about solving the mystery / trying to rescue the other person.

She tries various ways to stay incognito, including "borrowing" her older sister's makeup and a wig to disguise herself, but at one point a doorman(?) says something to the effect of "how could I forget you in that getup?" and she realises that her disguises are actually having the opposite effect.

When she eventually meets the mystery person hanging the signs, (view spoiler)[she learns that it's a girl her age, who uses a wheelchair and can't get out much, so she's also bored. She's been hanging the HELP signs because she knew the main character was watching—she's on a slightly higher floor, so she has a better view into the main character's apartment, and she also has binoculars—and wanted to pique her interest. The story ends shortly thereafter, with the impression that now they each have a friend and will be less bored and lonely. (hide spoiler)]

I have vague associations of a name starting with V—Vicky? Vickie?—for the main character, but it's possible that that's a red herring.

Approximate pub date:
I read this around 1999, no later than 2004. My copy was secondhand, and it could have been published earlier (80s or 90s, not likely before).

Physical details:
It was a slim mass-market paperback, not more than 200 pages. There was some sort of illustration (not a photo) on the cover (maybe the main character, but I'm not sure; I think she had curly hair, which may have featured), with a sort of...frame?...of a floral(?) pattern that was primarily a darkish green.

I haven't had any luck with WorldCat or Google Books, but I'm not highly skilled in either of those. Would appreciate any leads!

[Becca]

Mystery on the Fourteenth Floor by Dorothy McKay Martin looks like a match. You were right about the name Vickie!

[Liralen]

Yes! Gosh, that was speedy. Thank you!