Wherever the script registered with [package unknown]
is programmed to look.  That is, this is completely
application configurable.

[MG] I think that's slightly misleading; [package unknown] is checked as a last resort, after [package require] has tried to load a package from the usual places and failed. (What the usual places are, I'm not completely sure - hopefully someone can fill that in...)
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[MG] Slightly related, you can check which files were loaded/sourced by a [package] by using [package ifneeded]:

 (User) 1 % package require Tk
 8.4
 (User) 2 % package ifneeded Tk 8.4
 load {C:/Program Files/Tcl/lib/tk8.4/../../bin/tk84.dll} Tk



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[Category Package]