Everything is evidential , svejk. The behaviour of the defendant gives a clue to what he was thinking. Curiously, that is why a man fiddling the books may not be guilty of theft. It may be that the rest of his behaviour suggests that he is not stealing; he is not hiding money or goods. That is when we charge him with what we traditionally term 'false accounting'. He may be fiddling the books but his reason may be that he is incompetent, and thinks that, if his books pass muster , he'll avoid getting the sack, so, realising that his books aren't accurate and don't match reality, he alters them or makes false entries so they appear to be right. So he's then charged with making the false entries, not theft
And clearly the man who hides the umbrella may be doing so because, when he appropriates it, he knows it is a far more expensive umbrella than his own; it all suggests that his claimed belief is not genuine. However strange the mistake, the test is of genuiness of belief, but, obviously, the stranger it appears to be, the less likely it is that his belief is genuine. A claimed belief which is manifestly unreasonable is not going to be sufficient
The manager who turns a blind eye to an overt, obvious, taking of the firm's property does not necessarily make the taker dishonest. It may be construed by the taker as implied consent, given with authority; otherwise why did the manager not do anything ? But you can rely on the jury, who see the witnesses, probably including the defendant, to say how likely it is that the taker thought that, rather than that the manager was dishonest and the defendant was being equallly so. They might think the defendant was acting innocently, but that won't be often !