Whenever a solution involves melt, pivot_table, or pivot, you can be sure that there is an alternative method using stack and unstack. The trick is first to move the columns that are not currently being pivoted into the index:

>>> sensors.set_index(['Group', 'Property']) 
           .stack() 
           .unstack('Property') 
           .rename_axis(['Group', 'Year'], axis='index') 
           .rename_axis(None, axis='columns') 
           .reset_index()