Hello and welcome- they do indeed appear to mysteriously vanish at times, but there's almost always an explanation. If your anemone is an RBTA it's highly unlikely it ate it, but not impossible; other more predatory anemone species, perhaps. Also, in a 150g tank, you probably have a fair amount of rockwork? They can be experts at getting in a nook or cranny- or being eaten by something residing there, such as a crab or starfish. Overflows and sumps are good places to look, and some are jumpers that our dogs or cats (or housemice?) may get to before we notice them anywhere on the floor. Sorry it's not turning up!
