Solution:

In our experience, digital current loop drivers typically turn off the current as completely as they can for a space state. Because the signal is usually an asynchronous serial signal, the receiving UART detects an error condition (break state) when the line is without current for more than one character time. This corresponds to the like situation in RS-232 when the line is held high for a long time. I believe that the designers of analog current loop, where the current in the loop is directly proportional to an analog reading, reserved any current below 4mA to indicate an error condition. Because the signal could be any current level at any time, they needed some way to identify a break in the line. In digital current loop, I'm sure most receivers have some minimum threshold of current that they need to interpret the data as a mark condition. Because most of these systems are old and proprietary, we have not found that there is any one standard as to where to set this point.