tuga
Legal Alien
There are these little fellas:
http://neatacoustics.com/iota/
http://neatacoustics.com/iota-alpha/
I own the alphas. They're certainly more extended and transparent in the treble than other speakers I've owned, albeit they were mostly all soft dome tweeter models of 90s vintage.
More extended means that the tweeter can produce sound at higher frequencies. Since most tweeters can go beyond the human audible range, that is probably not what you are experiencing.
Generally when a tweeter attracts attention this means that either it is producing distortion and/or that it has been poorly integrated and as a result is "playing louder" than the other drivers.