Like I said, I don't ever turn her out with a rope halter on (or any halter, for that matter). On this particular day, I just let her off the lead line while in the arena with me so she could eat while my friend and I were 10 feet away stacking ground poles. If any thing were to happen to her, Joel and I were right there, and as you may be able to guess, she's not the type to panic anyway. For desensitizing I usually would have her on a lead line, but this was a spur of the moment thing when we realized while making a lot of noise folding the tarp that she didn't even seem to notice it. I understand your concern though, JDI.
That seems to be her reaction to everything, a great, big, "No big deal!". I was kind of hoping for something a little more fiery (she's supposed to be my next barrel racer when she's old enough), but I'm starting to enjoy her temperament. It's gotten to the point of everyday going to the barn and trying to find something new I can scare her with (I follow the Clinton Anderson method of 'heart attacks are free - let them have one!') to further desensitize her and every day it's just "oh wow, a giant bouquet of balloons? Can I eat them? Oh, no, well then no interest to me. Carry on waving them around!".
I've spent most of my life riding horses. The rest I've just wasted.