I just finished a three week period with a Holter.
The only way for it *not* to be noticed, in my opinion, is if you dress in separates and hide it with scarves and jackets. It's the size and shape of an 80s walkman. With wires. Can you tell I hated it?
After two days, I got tired of it and of hiding it and just wore it openly. If I had a dress on, I wired through the neckline and wore it on the neck carrier. If I wore a skirt, I ran the wires through the bottom of my top and connected it to the hip carrier. That was less obvious.
I still hated it so very very much.