Related story: I had a weird fermentation where I set up a blow off tube and I saw no bubbles for a few days, so I took a gravity reading and it had dropped, so it was definitely fermenting. I switched to an airlock and got bubbles. People said I must not have had an air tight seal on my lid when I had the blowoff on, but when I pressed on the lid, I could get the water/air level in the blowoff tube to move, so it was definitely air tight. Who knows what happened, but it definitely fermented.