df = pd.DataFrame({'A': [1, 3, 8, 5], 'B': [3, 3, 2, 8], 'x': [0.4, 0.3, 
    0.5, 0.8]})
unique = pd.melt(df.reset_index(), id_vars='index', value_vars=['A', 'B'],
    value_name='SingleNumber').drop_duplicates(subset='SingleNumber', keep=
    False)
Find position of value, that occurs only once in dataframe