I've never gotten a regifted item that was to my taste or of value. I've only gotten items that were clearly given to me because the intended recipient didn't like them and didn't want to throw them away for themselves. Think old bath products (I hate perfume), freebies from cosmetic counters, ugly figurines, etc.
I really doubt that someone who thoughtfully looked at a gift and decided that a friend would love it, would be careless enough to not open it. Not to mention I was raised to always look for a card first, opening a gift without reading the card was unspeakably rude.
Yes, this first part is exactly how my cousin's gifts I mentioned upthread were. One was a pack of body wash you could buy in any drugstore, which is fine, but I know her well enough to know she wouldn't have spent the $$ to buy it for me, so it had to be sometning she was re-gifting. Which I promptly GAVE, not regifted to another friend, asking if she'd use it, since I wasn't going to.
the second part, yes too. In the past, when I've re-gifted something I didn't care for or couldn't use, I was very careful to make sure nothing was left on it that identified it was being a re-gift. I think in cases where something is left behind, it says to me that the giver couldn't be bothered and just gave it.