Overly simplified, macrophages recognise pathogenic patterns and endocytose anything that matches them.
That also works on bacteria, which are quite often very mobile. What if a bacterium was just randomly twitching around when a macrophage recognised it - would it be possible for it to "swim away" (as a reaction to the macrophage or just randomly)? Or do macrophages employ some sort of fixation mechanism to keep anything they recognise close until it is engulfed? Or is engulfment in itself just so quick?