The shirt is a rather dull and nondescript shirt that is easily missed. The shirt is so ordinary that it hardly rates a second glance.
Wearer of this shirt can walk unnoticed into almost any even/gathering/bank and perform just about any action without being noticed. Only extremely out-of-place action will get the wearer noticed at all.
When worn the shirt conveys its ability to go unnoticed to its wearer. People wearing this shirt are often overlooked or even totally unnoticed. The shirt is magical but its magic is so successful that only the closet inspection about the shirt will give off any information about its nature. The shirt will allow its wearer to get away with a lot of actions but anything truly obvious will get noticed.