No one should give out their UPS Employee ID Number, for the same reason you should not give out your Social Security Number. Indeed, laws have been passed to allow people to refuse to use their SS# for anything other than Social Security purposes.
Your UPS Employee ID Number can be used to access your personal and financial information on UPSers.com. Your profile and your last two year's of pay stubbs are a goldmine of information that can be used to obtain even more information.
Although UPSers.com does require a password, lots of new hires as well as people who don't have computers have not yet logged on and so their default password is determined by a publicly known formula.
Additional information can be gleaned by looking at seniority lists posted on center bulletinboards. This info helps in constructing the default password. Some other passwords can be guessed.