Do you have any "exposed" powerheads that the anemone could have gotten sucked into? If so, then I wouldn't doubt it. Does your water have an "odor" to it that is out of the norm, like maybe smelling "sweeter"? I observed that the several times (yes, several) my anemone lost a battle to a powerhead, the water smelled differently. If I were you, I would do a water change immediately, becuase if the anemone did get sucked up into a powerhead and was shredded, the "parts" in the water should start to affect your ammonia levels pretty fast. And if for some reason your anemone is "hiding" or had retracted completely because of a defensive reaction, chances are that there is either something in the water that is bothering it, whether it be chemical or animal. If it is chemical, then you would still want to do a water change. I have seen my 14" carpet pull itself under the sand bed to the size less than my palm, so your anemone can be anywhere if it trying to hide.