Hi Charlie,
Looking at the last error history (click the "history" button) is a good way to see what is happening when using mcTouchOff. The guys at Mach have been very helpful by sending messages there when things are happening in that script, such as a probing move, setting offsets, retract moves, etc.
The command that Craig mentioned, G91 F5 followed by mcCntlEstop, looks like it is from the CheckProbe() function in the script. Look for the "CheckProbe failed" comment. It's also sending a message to the error log so it will be telling you something in that last error history.