Saw some interesting Honda behavior this morning. Temps were near freezing (maybe 35F) and I was headed down the big hill right after I leave my apartment. I usually handle this by putting it in third or fourth gear to control speed with compression braking while letting the engine run somewhere above its high idle. (High idle after dead cold is about 1700RPM, I had it turning at about 2,000RPM.) At the bottom of this hill I clutch in and roll to the stop sign, which usually means the engine is clattering away at high idle again until I drive off. But today it didn't. Instead, the idle dropped down to 500RPM and just stayed there. After giving it some gas to get away from the stop sign it resumed normal behavior.
Anybody want to guess why the idle dropped to 500RPM for no particular reason? I'm just curious, as there isn't a CEL and the car ran just about perfect after that.
In cold weather, my Civic often exhibits strange idle behavior - sometimes surging, sometimes sticking high, sometimes dropping low like you have described. You can check for vacuum leaks in vacuum lines, but you probably won't find any (I haven't).