areas where one might get mugged or encounter unsavory characters, or perhaps simply areas that don't look "nice" (by the standards of the hotel guests)

From the Urban Dictionary:
http://www.urbandictionary.com/define.php?term=sketchy
1. Sketchy

1) someone or something that just isnt right.
2) the feeling you get the morning after usuing a lot of drugs, most commonly associated with extacy.
3) something unsafe
4) someone or something that gives off a bad feeling

2. Sketchy

1. Adj. - Describes a person that has taken too much Speed and has become untrustworthy and unpredictable.

2. Also used to describe a place or action that is worrisome or unconfortable.

3. Sketchy

Iffy. Questionable.

4. sketchy 103 up, 29 down

This expression was first coined in reference to hastily drawn out, or sketched, building/construction plans, which were uncertain at best and disastrous at worst. The overall thread of meaning is the same, but now applies to morality/legality. On the overall continuim of goodness, things progress, in order: safe, sketchy, shady, outright criminal.

Getting wasted and gambling away half your paycheck in Nevada may be legal, but its definitely sketchy.

5. sketchy 67 up, 3 down

creepy, iffy, fairly unsafe, an air of uncertainty, not kosher, and just generally something or someone that you don't want to be associated with (or really do want to be associated with, depending on who you are...)

". Sketchy 340 up, 42 down

" 1) someone or something that just isnt right.
2) the feeling you get the morning after usuing a lot of drugs, most commonly associated with extacy.
3) something unsafe
4) someone or something that gives off a bad feeling"
(From: http://tinyurl.com/2zdoey )

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Note added at 6 mins (2007-02-22 22:47:41 GMT)
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Oh dear - I'm sorry - the weird bit at the top is the voting on the dictionary entry I copied and posted in.

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Note added at 8 mins (2007-02-22 22:49:32 GMT)
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The link gives you a good explanation of the word and it's meaning(s), e.g.

"This expression was first coined in reference to hastily drawn out, or sketched, building/construction plans, which were uncertain at best and disastrous at worst. The overall thread of meaning is the same, but now applies to morality/legality. On the overall continuim of goodness, things progress, in order: safe, sketchy, shady, outright criminal."