I can see a woman saying she considered finding that on your computer cheating...not in the traditional sense, but lying. Same goes with her just finding it on you.
I know a lot of people get into chastity as a form of control, but not men giving them control, men taking from them:
1. We never have sex anyway
2. I'm tired of rejection
3. Just give her the keys and she will tell me when she is ready.
4. Will start by setting rules, like at least once a month release for sex, time limits etc.
5. This guarantees you will have sex more than you do, never get rejected, and forces her to feel obligated to have sex by a certain date.
Chastity can feel forced on to someone, and lets face it, in general she already has control. The only thing you are bringing her is being responsible for your pleasure...she already has control over hers.
So as
@Jasmic68 said, her just discovering it on you, I think would end negatively.
Talk with her. She's your partner. She deserves to know who she's sleeping next to. Talk about masterbation, how often. Talk about her masterbation. Talk about how much you are turned on by give her control. Don't mention side effects like being attentive,treating her better, or cleaning more...she will say she shouldn't have to lock your dick up for you to do that.
I wish ya the best, best part of chastity is how communication becomes easier, I suggest you try that.