My friend K. went on a trip last week, and left a t-shirt behind. He didn't even notice it was missing until it turned up in the mail, shipped to him by the owner of the inn where he stayed.
Of course, the inn owners are getting a nice, hand-written thank you in the mail.
Although these days you'll hear that you're "technically" not required to send a thank you (because gifts should be given in the spirit -- yeah, bullshit), good manners dictate that when someone goes out of their way for you, or buys you a gift, you take about 2 minutes out of your life, and send a thank you note. Hand-written. Not pre-printed photocards -- those stupid things are tacky and, no, they don't get you off the hook.
I was filing receipts this weekend and came across proof of multiple baby gifts I'd bought for random procreators--you know, casual friends and acquaintances. Not one had sent me a thank-you or even called to say thanks. So unless you're a friend or relative of mine and you're pregnant, no more gifts from the Bitch.