As a token of my appreciation, I gave a co-worker a gift card to Starbucks. They accepted it and said they would love to stop by and get a coffee on the way to work one day. A week later, I found it in the original envelope with a note that said, "Thank you for the gesture but you don't owe me anything, thanks" and they signed their name.

I am extremely confused. I could understand if you didn't like Starbucks and had no use for the card. However, why accept the card and say you could use it, then hide it in my office mail box?

This person is head of our department and has to cover when people are out. It just so happens I had to be out for a week due to illness and doctors orders. A lot of people have been out sick lately and it just makes me wonder if this person is mad about having to cover.

Was I wrong in offering a thank you gift? I did it because I was genuinely grateful. There was no other motive.

It may be against company policy. She/he is in a leadership position and that may be another reason. I sent a gift card last week to a very nice animal control officer who helped me over the phone. I had it returned to me in the mail. I'll assume it was against company policy.

If it isn't a matter of policy then iss it possible that after accepting your gift this person thought it over and felt badly about having accepted it.

I know that sometimes when someone gives me a thank you gift I feel that I shouldn't accept it because we don't always have to be thanked for good deeds; however, I must admit it seems they picked an odd way of returning your gift. Talking to you in person would have been more appropriate.

She probably mentioned the gift to another employer or her supervisor, and was cautioned by one of them in some way or another.
For whatever reason she made the decision,and it is her decision (or maybe her boss made the decision) but she handled it well in my opinion. Forget it and move on.