Is it fermenting?

I'm in a similar situation to the OP in this thread. I think I under-pitched my yeast. Can you help me figure out if that's the case?

Here's the recipe I followed. I didn't measure the OG before I threw it in the fermenter and pitched. I have had good luck with just popping the nutrient pack on my Wyeast starters at least 3 hours before pitching.

This time I forgot and didn't pop it until about 2 hours. It had started to swell, but not a lot.

I brewed Oct 26 in the evening. Oct 27 the foam did (I think) turn into a krausen. I do have sediment on the bottom. I did see a lot of tiny particle activity when I looked close. I did not see any bubbles out of the airlock. I checked my lid (I'm in a Fermonster) and it was tight. I check the seal around the stopper and it was fine.

On Oct 28, I read up and decided I needed to check the gravity. It was 1.040. This beer is supposed to start at 1.070 and go to 1.015.

Here's my theory. Check me. It's on it's way from 1.070 to 1.015. It's fermenting. I had a leak in the o-ring under the lid that I couldn't see, so the excess air was going out of that instead of through the airlock. I just need to hold tight and I'll have beer.

I just don't understand why I haven't seen the airlock activity at all. I haven't experienced that before and this is my 9th batch of beer. Thanks.