Actually, we don't even know what part of the Great Valley he's in. Could be near Saurus Rock for all we know. There's not too many guys there, or else somebody might have noticed those two Sharpteeth wandering around.

Anyway, my villain is guilty of speaking his plans out loud, though he doesn't shout them (though I had thought of him yelling them too loudly, thinking he was alone with a few of the heroes, only for others to come and tell him off for what a creep he is!).
However, my villain has the added luck of looking like a Flattooth so he can get away with more.
On the flip side, considering that Sharpteeth have their own language, nobody might be able to understand the bad guy here in "wrongly accused" anyway, unless he's bilingual, and it took the Great Valley residents some time to figure out Chomper's parents were in the Valley, even though they knocked over a few trees and roared a bit.
It's not beyond the realm of possibility that a villain who's not too near and only speaks in Sharptooth might go unnoticed.
Heck, considering that Chomper, in Great Valley Adventure, was only one tree line away from Leaf Eater view and was wandering around chasing bugs, I'd say the Great Valley residents aren't too on the lookout for threats within the confines of the Great Valley.
(Ozzy and Strut were able to get around pretty easily too, even singing the Egg Song in the Great Valley itself and they didn't fear getting overheard by angry mothers and beaten into a pulp.)
Also, Pterano was able to sneak in despite being banished for life. Security isn't the best in the Great Valley so don't count out that such a villain could talk to himself without being spotted.
(Also, perhaps the villain is near the very spot where his brother died, which is at the Great Valley gate. He'd be high up, and could spot any enemies before they saw him (unless they came from behind, and they'd have to fight him if they did) and hence can make some noise and get away with it. In fact, him being near where his brother died actually makes a lot of sense, could still be in the Great Valley, and be away enough where few Leaf Eaters, if any, would be, and he could see them coming in time and plan an attack if they came, unless it was in numbers, and he could still see THAT coming too and run for it.)
Plus, with the exception of the execution of the two knocked out Sharpteeth, it seems more likely that Great Valley residents will just kick out adult Sharpteeth rather than kill them, as was the case in film II, film XI, and twice in the TV Series. He might not be in danger of death unless they have a desire to bury him alive.