I would never leave a halter on an unattended horse - no matter where it is. If you can't "catch" your horse, you haven't trained it very well...
my only exception to always being halter free is older untouched horses being halter broke. I have one right now that will have a light, well fitted halter left on for a week or so after I get it on the first time. Trying to rehalter a fearful 18hh 5 year old every day isnt much fun. Much nicer to leave it on for a few days.