There's a couple of likely possibilities for how the value of Pi is included in the code -
1) It is calculated using trig functions. e.g.
4.0 * atan( 1.0 )
2) It is stored as a floating-point constant. In hexadecimal representation these could be -
0x40490FDB // 32-bit floating point
0x400921FB54442D18 // 64-bit floating point
However, you need to be aware of compiler optimisations, including constant folding.
Even if the source code uses my first possibility, the compiler may optimise it to a floating-point constant. (i.e. my 2nd possibility)
Then, if the value Pi is only ever used in contexts where it appears in calculations with other constant values, the compiler may perform that part of any calculations at compile time and include only the result in the object file.
pi = 4.0 * atan( 1.0 ); // may become = 3.1415926536..
circumference = 2.0 * pi * radius; // may become = 6.2831853.. * radius
degrees = radians * 180 / pi; // may become = radians * 57.2957795..
result = fourier_integral / (2.0 * pi); // may become = fourier_integral * 0.1591549..
So, in summary, you might not find the value of Pi at all and may have to search for likely related constant values.