Saw this tonight, pretty surprised this is the first time ive seen it as i havnt added much since october and i dip everything pretty thoroughly. It does look a bit like some keyhole limpets that i bought 4 years ago but i havnt seen one in a year or so. Anychance this is an offspring? Doesnt seem dark enough. Or maybe a stomatella?
Its maybe an inch or so long.

Its maybe an inch or so long.
