This is not the first time, but now I am really sure of what happens even if I do not know why.
I add a new parameter to a Method and I started to see - in the Navigator - the Method name WITH the two parameters.
To hide the parameters, I only had to close the Methods entry in the Navigator and re-open it.
OK: this happens in 2015r1 (not tested with 2016r3) and El Capitan. But this is relatively new for me.