It's great that Belle is finally seeing Tom for what he is and standing up for him but FFS will somebody listen to her. I know Sam and Lydia aren't the brightest but they were completely taken in by her and as for Amelia sticking her nib in, in the pub, when she knows bugger all about the situation.....
If I remember correctly, when the locks were changed, the locksmith originally took the key away with him and Belle was going to collect a spare in a couple of weeks. Instead, Tom took the key back from the guy. So if Belle calls to get the spare key, they'll tell her that her husband said she'd changed her mind and then she'll know he kept a key and has been letting himself in.
I've said it before but there is no way he'd be able to let himself in without anybody noticing. I live in a similar place and whatever happens, somebody always sees it.