Just wait & see if the light comes back on at all. I doubt that you missed a code by watching the blinks, it is really fairly obvious.
A brief bit of acting up, caused by any multitude of things, can make a light come on. A few seconds of misfiring can do it. Water in the gas, or a dirty plug that burns itself clean again, etc. It may never happen again.
I don't know about Nissan computers, but after a certain number of starts, the code will self-erase in many computers if the problem isn't detected again. So if one were set, it could be gone after, say, 100 starts.
Power loss in the mountains? Of course. You lose 11% of HP at only
3,000 feet, and it gets much worse from there. The power loss is logarithmic; the higher you go, the faster you lose oxygen. It's about like having compression loss in every cylinder.
Your cold start problem may be related to cold temperature as well as the altitude.
Some cars are far more affected by altitude than others. I took a Sebring up to 10,000+ feet 5 years ago, and it could hardly accelerate at all even on level ground. It was truly pathetic. Last year I made the same trip up the mountain in a late model 4 cylinder Altima, and it did quite well in comparison, but you could definitely tell that
70% (give or take) of the power was gone.
I was in Colorado a week ago, in a Grand Caravan rental. Got up to
11,000+ feet. Had to flog the heck out of it going up steep grades at
50-55 mph in 2d gear a lot of the way.
So anyway, your drivability complaint is related to altitude IMO. And if there really is something gone wrong, your light will come back on, trust me. Just wait for it.
JM