If you threw me in a vat of beer and then noted the level, then I drank 3 liters of the beer then checked the level, wouldn't it be the same? My mass would grow by the equivalent of 3 liters of beer but the level would stay the same. After a while though, I would start releasing gas, liquid, and perhaps solid waste. The level might change but its hard to say by how much. The liquid and solid waste shouldn't effect the level but the gaseous excretions would. The gas that is trapped in the liquid would make it less dense so it would expand (I think) but some gas should also escape, increasing density and decreasing volume. Judging from all the gas I release when I degass my mead, quite a bit of the gas stays in solution, which should increase volume. So I think maybe Jim is onto something, but its the increase in gas, not increase in the volume or mass of yeast, which comes at the expense of it ingesting and using other matter. Nonetheless, that still means fermentation is happening.