One can both flee and fake, and I think Sauron has used this method before: allow the enemy to think that they have struck terror into your poor, weak, pitiful heart, don't let them know how strong you really are, and then only later, show your real power. As honey-tongued a liar as Saruman showed himself to be, he was an amateur compared to Sauron, who managed to convince the King of Numenor that he was truly surrendering to him and was so sufficiently defeated and demoralized that it was safe to take him back to Numenor as a "hostage." It didn't take long before Sauron was able to corrupt Ar-Pharazon and his court, and it wasn't long after "fleeing" Dol Guldur that Sauron openly took up residence in his real stronghold, Barad-dur, and began making trouble on a much grander scale. I'm blanking a bit on some of Sauron's activities in the First Age, but if I'm recalling correctly, he did some "fake fleeing" back then, too. He knows when it's foolish to keep on fighting -- when it's time to employ the maxim "he who fights and runs away lives to fight another day" -- and if he can fake his enemies into thinking he is fleeing because he is near-mortally wounded or vastly overpowered, even better. That would lull them into an at least temporary sense of relief and security, so that they will not be expecting retaliation from him any time in the near future, and thus may be surprised and caught unprepared when it comes.
My nickel's worth, at any rate.
