Hey, BeShy
Sorry to arrive rather late to this discussion. I have a keen interest in men's kinks and fetishes and my research has been both academic and "practical".
I haven't read every single post in this thread so apologies if I repeat things that have already been said.
Firstly, discovering that a man owns a chastity cage indicates that he has an interest in having his penis locked up so that he can't get an erection. This isn't as daft as it sounds- a sort of low key arousal that goes on for hours or days is allegedly better than a few seconds of pleasure followed by the male post orgasm "low".
It does not indicate that he is gay, submissive, strange, perverted, or anything else. It absolutely does not make him a man to be scared of (unless you are scared of him for other reasons).
Additionally, it does not automatically indicate that he is in another relationship. Many men carry a fantasy about being locked and controlled, but they enjoy the fantasy by self locking. The reality would be way too scary for them.
If you want to start a discussion without being found guilty of snooping in his wardrobe, your best bet is probably to discover the cage when he is wearing it. If you have the patience, check whether it is present in his wardrobe. If it is not there, he is probably wearing it. You are then in a position to discover it by accident, brushing against him, or commenting on the unusual bulge in his pants.
You then have a number of options depending on which way the discussion goes. If he is seeing another woman, or indeed paying a dominatrix to hold the key for him, you have said that those might well be dealbreakers in your marriage. If he is self locking, that might still be a dealbreaker, but on the other hand, you might be inclined to discuss and explore together. You might ask him how he would feel about you holding the key (and the spare keys- they always have a spare or two).
Being a keyholder is huge fun if the person locked is a man that you love, or at least fancy. For a man that you don't love or fancy, the appeal is less, but there are still advantages to being his keyholder. Firstly, it prevents anyone else from being his keyholder. Secondly, you will discover the most amazing change in his attitude towards you which will invariably be positive.
Good luck, and try to have fun with this, whatever the eventual outcome for your marriage. Tell us how it goes.
Alex :-)