I boiled and pitched a low-gravity (~1.033) Scottish Ale this past Sunday.
The wort was cooled via immersion to under 75° F and then aerated via a whisk-on-drill contraption before pitching the yeast. The house has stayed at a consistent 75° F the whole time, within range of the yeast's comfort zone.
As of this morning (Wednesday), there was no activity to speak of. Despite verifying the airlock and seals were all tight, it was apparent no pressure had been generated. I removed the airlock and found neither krausen or any yeasty smell--it looked and smelled the same as before I lidded it on Sunday.
When I came home this afternoon, I was about to attempt a re-pitch when I found something rather different. The airlock was bubbling away. I popped the top I found quite a healthy-looking (if a bit dark) krausen, and a definite yeasty smell. Perhaps it wasn't quite the smell I expected (I'm not used to smelling it mid-fermentation), but it certainly smelled more of beer than of wort.
So what was going on the past three days that made the yeast take so long to kick in? Do I need to worry about acquiring an infection during this time without alcohol, and particularly having now opened the lid twice (I'd never opened it before bottling time in the past)? Or should I just relax, not worry, and drink a homebrew?