Has a bankai ever been silently activated? To activate a Bankai or its ability, is the user required to perform some verbal command activation? 

For example, Ichigo often yells


  Bankai: Tensa Zangetsu


However, I'm pretty sure that I've read some silent activations, but I can't remember for sure.

So has a Bankai ever been activated without a vocal command? 

Note that this is not a duplicate of Can a mute activate a zanpakutou, as that question asks whether it is possible. And this question asks if a silent activation has ever happened in the series.
Yes, it has.

After re-reading the manga, this happens twice. Once when Ichigo fights Ulquiorra for the first time in Las Noches





And again in his fight against Grimmjow