I've never heard any Americans say "thank" instead of "thanks" or "thank you."

I have never heard just "thank;" only "thanks" or "thank you." Perhaps a certain group of friends have started saying it with each other, or perhaps they say it as a joke? Edit: It is also possible you may have heard someone use the word "thank" in a longer sentence. "To thank" is a verb meaning "to show gratitude." So, you might say: "Please, thank Mr. Wilson for his hard work!" Did you perhaps hear something like this? Let me know if you are still confused.

This isn't an actual offline thing. On internet message boards and blog sites (particularly tumblr) and facebook groups one often finds people (usually teenagers I might add) who say "thank" as a 'memed up' version of saying thanks. Example dialogue: Mark says: "Here's that link you were searching for: https://www.whateverwebsite.org." Joan replies: "thank" Mark responds: "np." (np = no problem.) It's a very teenagery way of speaking and I'd recommend avoiding it as a second language learner.

Did you see this online? Because the only usage I can think of (other than as a verb) is "Thank mr skeltal," which is a meme based on an image of a skull playing a trumpet. Someone found the image and posted it on a discussion board and said, "Thank mr skeltal," and then there was a whole thread of people saying, "Thank mr skeltal." The image became popular for absolutely no reason. Because of this, some people on social media use "thank" this way as a joke. It's intentionally grammatically incorrect. Here's the image: https://i.ytimg.com/vi/mS2sP9ENM1M/hqdefault.jpg The Internet can be weird.