If the "library" was included in your sketch enclosed in quotes e.g. #include "myLib1.h" then you should find it in the same folder as the sketch.
If it was included with angel brackets, e.g. #include <myLib1.h> it should be in the standard library folder. You should be able to find the path by File->preferences in the IDE.
mikedb:
Is there a way to see what files is added for a sketch except the INCLUDE files shown in the sketch itself?
Do this
File > Preferences > Show verbose output during: > compilation (check) > OK
Sketch > Verify/Compile
After the compilation finishes, examine the contents of the black console window at the bottom of the Arduino IDE window. You will need to scroll up to see it all. You will find the path of the libraries somewhere in that output. You can copy and paste the output to a text editor to do a Find operation on it to make it quicker to find what you're looking for.
Who cares about colors Actually I do, but the only ones that I care about are the ones in the output window when I compile I never knew that notepad had colours except for black and white (if those are actually colours).
Anyway
The normal locations
Standard Arduino libraries in C:\Program Files (x86)\Arduino\hardware\arduino\avr\libraries and C:\Program Files (x86)\Arduino\libraries
3rd party libraries
C:\Users\YourUserName\Documents\Arduino\libraries
Your own libraries
C:\Users\YourUserName\Documents\Arduino\YourSketch and C:\Users\YourUserName\Documents\Arduino\YourSketch\src; you might not have the latter as it needs to be created manually.