I've never actually heard it had to be hidden. (That doesn't mean it might not be advised, just I've never heard it.)
BUT ... sometimes I use a 33-knot continuous rope during service, but it wraps across my palm and is mostly hidden, and I generally cover that had with my other one if it's visible. And when I use a 100-knot, which can't be hidden, I keep it under a shawl. So ... that thinking is just there.
One thing I have heard, a layperson is not to wear a long prayer rope tucked in the belt as a monk does. Because that would be prideful.
I don't think the concern is quite so much directed against being seen praying in a completely secular setting, particularly one actually hostile to the faith. But doing so in Church or to be seen by other Christians would be the biggest problem, I would think.