People hide things from their spouse when they know their wrong, or if they are embarrassed of their behavior, or they think you will judge them.
I don't blame you for being upset but I wouldn't make too much of a deal to him... how you handle this will affect whether or not he will open up to you or not.
You left out "shame". Some folks are raised with a lot of shame when it comes to sexuality. Not to get weird here but almost everyone would not have sex in front of their children. For others even open affection in front of others is a no no. I know people that have smoked grass all their lives but would never tell their kids about it, even their adult children.
What I'm trying to say is that everyone has a set of boundaries they establish for themselves. Secrecy may even add to a persons excitement when they masturbate while others openly do it in front of each other for extra excitement. "I" know their adult kids get high and "they" know their kids get high but they never talk about it to each other.
I see you have been a member for a period of time. I'm sorry, I just joined so I have not read anything about you and your relationship before. I find that mates generally know their partners pretty well unless they really don't want to.
I see that a lot of times a posts comes out followed by details that extenuate or exacerbate everything said before. If you can do it without causing shame or embarrassment for him simply ask about it. If there is a problem best get at it right away. Afterall it's not a secret bank account.
BTW: I prefer to masturbate when my wife is out of the house. I have no idea if she does or not but it's her business. She is perfect in every way. Something like that is not a problem.
Jealousy requires active participation.