Seriously, I can't think of anything to worry about. The only thing that BA miles can be "spent" on is BA/OW flights. You can't transfer them to other programmes, you can't get rid of them in any other way.

And to be honest, we all check our accounts frequently enough to notice if something had gone missing. Then all you would have to do is cancel any booking that had been made.

Can anyone think of a situation where

a) regular check of balance
b) vigilance

would not mitigate the potential loss of any miles?