Generally speaking, a postcard is not considered commercial use. I sell stock photographs through agencies, and postcards are not considered "commercial" as far as can see. (You easily recognize commercial use by the price tag in the Rights Managed licensing model).
The grey area is book covers: do they promote the book? In case of a postcard, the buyer is buying a picture itself. Calendars are also, as far as I know, considered not commercial. Mind you, there are such things as commercial calendars (Playboy comes to mind for some reason), and commercial postcards (such as those promoting hotels, fairs etc.). I'm talking postcards and calendars without logos or promotional writings.
I would not be sure, in both cases, that the person is "incidental". In both pictures there basically is one person and a prominent sign "Guard on duty", and it is implied that the person portrayed is said guard on duty. So I would say that it could be argued that both persons are actually the subject of the picture, and that - stretching the concept a little bit - might be considered to fall within the ritratto ambientato genre, the "portrait of a person in his environment".
If the person was recognizable that in my opinion would require a model release for commercial use. What would make the situation different is if the person is not recognizable but again, if the commercial use is made in an environment (such as an hotel nearby) where the image would clearly recall the person and make a clear and understandable personal reference to him, even in silhouette, that would be another matter.
In short: according to my understanding, you don't need model release for non-commercial use, that's a general rule;
commercial use of these images is a bit of a tricky situation and the need of a model release cannot be totally excluded.
Something that is often overlooked is that the person might feel entitled to compensation if a non-commercial use is made of a picture which damages the reputation of the person.
If one year there is an abnormal number of deaths among Californian beach-goers, and a national scandal erupts about how ineffective are life guards in California, and a Californian newspaper writes an article entitled "Why do we have the worst life guards on earth", and a life guard is depicted like in this case, recognisable by his pairs, parents and friends, and the assumption might be made by some readers that the life guard portrayed is actually one of those who are accused of scarce behaviour on duty, then the person might actually sue because his reputation has been tarnished within his social environment.
All this is of no big interest to the photographer who is not a publisher. The photographer clearly states that he has no model release. The publisher evaluates the picture, the circumstances in which it is published, he gets the profits, and he pays the compensations if any.