polopp

Thanks for your detailed response. I'll have to be more careful in the future I guess 😕 Maximum sneaky indeed!
I've emailed support but they're taking forever to get back to me. However I've read the T&Cs thoroughly and it seems from the wording that I didn't use the bonus cash as it's only playable when I play through my real money deposit. So I think I'm in the clear. We'll see.  

There have been some cases of players getting bonuses credited into their accounts whilst they were playing with their cash money (no bonus taken during deposit), as in your case. Some casinos can be real tricky and they like to dump in bonuses into players account during play. The reason for casinos doing this is rather unclear, but I see it as a sneaky trick to put winning players into a sudden lockdown.
At some casinos though, a message would pop up on the screen, indicating that you have just earned a bonus, usually upon reaching a certain VIP level or ranking. Since you are playing with high stakes, I can imagine that you would reach a certain high level quite quickly and would therefore earn a bonus. The thing is, some bonuses need to be activated first, but some don't, and these automatic bonuses are the culprits that usually put real money players into unwanted situations.
Any bonus can be cancelled. Some can be cancelled before accepting it, others would need to be cancelled manually, either by the player or via live chat assistance. But once a bonus gets credited into your account, then bonus terms and restrictions automatically become active. Very sneaky indeed, right? 
Have you tried getting some clarifications from the casino's support?