Just an interesting anecdote...

So I brewed an ESB on Monday 1/18. It was 10.5lbs Maris Otter, .5lb Crystal 20L and .25lb Crystal 120L. I mashed at 151F for 60 minutes. OG at 65F was 1.063. 85% efficiency! Pitched a decanted 2 quart starter of WLP002. Set it out to ferment at 64F ambient temp.

I saw very subtle (read:none) airlock activity for only one day. I thought the ferment might be really slow or stuck.

I checked the SG today thinking of moving it to a warmer location, and to my surprise, it was down to 1.013!

Another 3 weeks in the fermenter, then to the bottle.

This is shaping up to be an epic brew (even if it is English)

Are you using a pail or carboy? I only get airlock activity about a third of the time with pails; the seal just isn't good enough. Even with a carboy, you must remember that the seal isn't necessarily airtight, nor does it have to be. As long as outside air doesn't come in (bringing contaminants) you're okay. The positive pressure created by CO2 generation means you'd need a pretty big "leak" for any outside dust to come in...