How to find from which row and column the value belong?
You can use pandas .isin() function to compare all the values of your dataframe to a given value:

In [10]: df[df.isin([6.9])]                                                     
Out[10]: 
   Height_1  Height_2  Height_3
0       NaN       NaN       NaN
1       NaN       6.9       NaN
2       NaN       NaN       NaN
3       NaN       NaN       NaN


Now, if you want to get rows and column directly from it use .stack() on it. So, it will be like:

In [11]: df[df.isin([6.9])].stack()                                             
Out[11]: 
1  Height_2    6.9
dtype: float64


The output is a series. This will work in case of multiple matches too where the output will be a dataframe.