yes, they are synonymous to some extent: slink to go or move stealthily or furtively (as in fear, shame, sneaking) : creep or steal along or away : retire ignominiously <slink behind an enemy> <slunk into a corner> search Yahoo with: "creep up behind" "slink up behind" (the quotation marks are important) and you'll see that "creep up behind" is much more frequent, thus I think it's the correct one in this contex, but ...
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