How to describe that an enemy can avoid opportunity attacks?

I’m the DM of my group. When we are playing I try to describe the effects and actions of enemies and npcs without spelling out exactly what they are to keep some mystique to the actions. Like describing that “the mage shoots a large electrical beam” instead of calling it lighting bolt outright.

I want to add a Flyby effect to a soon to be boss enemy, the description of Flyby is

Flyby: The [creature] doesn’t provoke Opportunity Attacks when it flies out of an enemy’s reach.

However I don’t want to make it seem as if the enemy is avoiding opportunity attacks just because, but neither I want to spell the effect out so they still have to think about it.

How then can I describe an enemy that has immunity to Opportunity Attacks without saying it exactly like that?