On the mainframe an unsigned data item with usage COMP-3, containing a sign byte of either 0F or 0C would pass the if zero condition test.  In Net Express or Visual COBOL only the field ending in 0F passes a compare test to zero.

Example:
       program-id. packzero.
       Data Division.
       Working-storage section.
       01 num-x pic X(3) value x'00000c'.
       01 num redefines num-x pic 99V99 COMP-3.

       procedure division.
       main.
           if num = zero
              display 'Value is Zero'
           else
              display 'Value is Non-Zero'
           end-if
           move x'00000f' to num-x
           if num = zero
              display 'Value is Zero'
           else
              display 'Value is Non-Zero'
           end-if

Results:
Value is Non-Zero
Value is Zero

On the mainframe the results are:
Value is Zero
Value is Zero

How can we emulate the behavior of the mainframe?

You can get the behavior you are looking for by setting the directive HOSTSIGNS on.

By default in MF COBOL only the 0F is a valid sign byte when the COMP-3 data item does not actually contain a sign character.

The 0C byte is used to denote a positive sign only when the data item itself is actually signed.

This is documented under Help-->Reference-->COBOL Language-->Part 1 Concepts-->Concepts of the COBOL Language-->Class and Category of Data-->Selection of Character Representation and Radix-->COMPUTATIONAL-3 or PACKED-DECIMAL-->Table 7:

Sign Convention in the PICTURE Clause Sign of Data Item Value Sign Half-character, in Hexadecimal
-----------------------------------

To make this behave like the mainframe use HOSTSIGNS and the 0C character will also be recognized.