From the YCM documentation.
YCM will display a short diagnostic message when you move your cursor
to the line with the error. You can get a detailed diagnostic message
with the <leader>d key mapping (can be changed in the options) YCM
provides when your cursor is on the line with the diagnostic.
You can also see the full diagnostic ...
It is not completely true that vim uses the beeps (or bells, as it calls them) only in errors.
There are many events that can trigger the bells, only some of them are errors.
You can custom the bell to ring only in errors (or whenever you want), by setting the value of 'belloff'.
All the error messages should be shown when they are happening, other things ...
So the issue wasn't guioptions, it was guiheadroom; I finally noticed this because while I keep my config files synced, the stub ~/.vimrc which sources them isn't, to accomodate local settings. One of which, on another system where I do not have the problem, is:
I added this here, restarted gvim and problem solved. From :help guiheadroom:...