I seem to have air in the hot water supply lines at any 3 of my sinks. It will clear from any one that I initially open.This is all new construction. A two story garage with 2 sinks on the second floor and 1 slop sink on the first floor. There is air trapped in only the hot water supplies. It takes about a minute for it to reach the hot water faucet, so I think it is air trapped near the gas fired hot water heater. This happens everytime I if I do not use the hot water for an overnight period. Two days w/o using it and it would have even more air to purge. I do not smell any odor from the air purging from the faucets. As far as I can tell I have no leaks in any of the lines.
If this is a U.S.A. pressure system, then if you had a leak, water would come out, air would not go into the pipe. And if it were just an air pocket it would flush out with the first water used and not come back again.