First of all, nobody in New York City looks anyone in the eye. Further, unless the next door neighbor or the doorman is you own lover, it is none of your business.
There are probably union rules that prohibit the staff from fraternizing with shareholders. But, unless that person is getting special favors or attention that others are not, those rules would probably not be enforceable. You may be one of the few residents aware of the situation because of your close proximity to one of the participants. And, for all you know, unless you have a hole in your wall, they may just be friends.
There are probably more affairs than you know going on in your building. Admittedly, these people are more discreet than your doorman but this is still their affair, not yours. Unless there is undue noise that is disturbing you, just keep minding your own business and looking down.
Dianne Ackerman is the new voice of reason behind Ms. Demeanor. She has lived in her Upper East Side co-op for the past 20 years and is the vice president of her co-op board. She is filled with opinions that she gladly shares with all who ask—and some who do not. Have something that needs sorting out? Drop her an email.