Sounds like people at work have already seen your hairy arms because, as you said, you have to wear a T-shirt to work. If this is done well, you are going to have some signs of redness around the follicle opening or white pustules perhaps, that others may notice. You may have patchiness as well after you heal. Two or three treatments sounds about right for a reduction only. Do you have coarse, dark hair on light skin?
Why not be honest and say I am having laser hair reduction on my arms because I don’t like having so much hair in this area? What are they going to do to you? Is this going to destroy any relationships with your co-workers? They are going to notice a difference anyway only if they noticed your hairy arms before. Just be honest and confident about this plan of change and people will know it and continue with their business with little care about your hairy arms. Some may even ask you for the name of your laser specialist so they can get their hairy butt lased. It’s funny how honesty begets honesty.
Dee