Strangely, an alternate wallet (which that unauthorised transaction did not appear) was able to transfer the full (correct amount of) BTC that I own into an alt coin successfully.
My BitCoin.com wallet (where the unauthorised transaction took place -- and which thus shows I have less BTC than I should) strangely also reflected that both (the unauthorised transaction, and my emptying of my BTC earlier to convert into another altcoin) were successful ~ which shouldn't be possible?
So my guess is that it was a glitch -- i.e. BitCoin.com wallet might have incorrectly read from a wrong address perhaps?
So all good now!!!
It was a nice timing to pull out LTC and BCH too anyway! ^~^
But cheers. I've saved the contact of Cyber just in case I or anyone else might require his service.