I've seen plenty of Infected do this
Where they crawl, in the crouch position endlessly in a direction and won't aggro or chase after you.
They also can crawl endlessly in the prone position, and not aggro, but they still hit you, if you get in their way, when they're in the prone position.
Stress test #51 on a NE server 2018-10-20
{F452473}