Something I've always been paranoid about is if the alcohol escapes through the airlock if I leave the liquid as-is for weeks after the fermentation have stopped.
Will I lose alcohol level?
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Not at all.
Technically, you may be losing an absolutely, immeasurably tiny amount of alcohol, but it would have to vaporize from the beer, absorb or condense into whatever liquid is in your airlock and then vaporize out the other side.
But practically speaking (which is what's really important) you'll never have to worry about this in an air-locked vessel. If it were open, maybe. But you should never leave your beer open to air anyway. So it really is a non-issue.
Alcohol Losses from Entrainment in Carbon Dioxide Evolved during Fermentation H. W. Zimmermann, E. A. Rossi Jr. and E. Wick + Author Affiliations
Research and Development, United Vintners, Asti, California Abstract
Total losses of alcohol entrained in CO2 in laboratory experiments were shown to increase with: temperature of fermentation, alcohol level of wine fermented, agitation of fermenting liquid, and presence of a pomace cap. As expected, the rate of loss of alcohol entrained in CO2 per unit of time reaches a maximum about the middle of fermentation, i.e. during maximum activity.
For four cellar fermentations, alcohol losses entrained in CO2 are reported as % of alcohol production at four instants during fermentation. The rate of loss of alcohol expressed in this way rises to a maximum toward the end of fermentation. No differences in losses of alcohol in CO2 were observed between the open and closed fermenters.
Total losses of alcohol by evaporation and entrainment in CO2 for laboratory fermentations af 26.5°C (79.7°F) were 0.65% for juice with 21° initial Balling, 0.84% for must with 21° initial Balling, and 0.16% for distilling material with 5.5° initial Balling. For plant fermentations at 25.5°C (77.9°F), the loss was 0.7% for must with 16° initial Balling. These losses are in good agreement with previous findings.
Does alcohol vapor escape via the airlock? Yes, it does; however, the amount that escapes is negligible in terms of affecting the alcohol level in the fermenter. If you go up to the fermenter and smell the top of the airlock as the primary begins to settle down, there is a very distinct scent of alcohol among the other aromas of breadiness, malt and hops.
Think about it this way, many many people, when primary fermentation has run its course, will rack the now beer from the primary fermentation vessel and into another fermentation vessel to do a "secondary" fermentation to allow the remaining suspended yeast to work a little bit more magic and more so to allow for the remaining yeasts and proteins to flocculate out and settle on the bottom. This stage, if one uses it, is also the time when one would add finings such as gelatin, isinglass, etc... to help force the clarification. Depending on the recipe, style and personal habits, the beer could be in seconday for anywhere between a week and a month! And even then the quantity of loss of alcohol is still negligible; even when it is a high SG and low FG recipe and therefore high in alcohol content. Personally, I have brewed beer where I did the primary ferment, followed by the "secondary" of 1.5 weeks and during that time I used gelatin to clarify and knock out of suspension remaining yeast and especially to help rid the brew of chill-haze proteins. In addition, I nearly always use Irish Moss near the end of the boil for that purpose as well (unless I am brewing a hefe-weizen, a witbier or some other naturally hazy style.) Then after that 1.5 weeks in "secondary" I racked the beer over some medium toast oak chips and let it sit on those for a couple months! Even after all that time, my finishing gravity did not change from when I tested it right before I racked it to secondary.
So, in summary, it is not enough to be concerned about. I forget who to properly attribute this to, but... Relax, kick back, have a home brew :)
It is incorrect to say alcohol will only vaporize at 174-178. The boiling point of alcohol is 173. It would be the same as saying water will only vaporize at 212. Of course water, like alcohol, will evaporate at lower temperatures. So alcohol and water can evaporate during fermentation, especially from a primary fermenter with no airlock.