It sounds to me like you just were very careless and wrongly assumed your wife would never notice the ATM withdrawals or credit card cash advances whichever you were doing. This is a good lesson for all hobbyists, always act like your SO is suspicious even when she isn't. Consider what she would be able to uncover were she to look, and be sure the answer is as close to nothing as possible. Unless your goal is to hurt her, you'd better keep this side of your life compartmentalized.
Your main problem now is going to be that your wife now has her antennae up regarding your financial behavior so any attempts to funnel cashflow to your hobby fund will be that much more difficult. Also be ready for her to be suspicious of your whereabouts now too as she is now tuned into potential abnormal behavior from you. As Mynx said, she now thinks you have an addiction and , in her attempts to help you, will most likely be vigilant in spotting any "backsliding".
In my opinion you should scale back the hobbying for awhile until things cool off. If not, then I'd funnel cash very slowly using a multipronged approach utilizing all the methods people have mentioned here plus any others you can think of.
Be careful with a separate bank account, if you ever happen to earn more than $10 in interest the bank has to give you a 1099-INT for your taxes which would be difficult to explain. Also funneling a portion of paychecks is a bad idea because the amounts going into your joint account and the amounts on your W-2 come tax time will not be remotely close, also your paystubs will show the trail.
The thing for me would be to do it very slowly and sporadically funneling small amounts from different areas at once. Things you can do (many have already been mentioned):
- Always get $5-20 cash back from debit transactions when possible, vary the amount depending on purchase totals and be sure your bank doesn't record them
- starting a hobby that costs money is the easiest way to get no questions asked chunks of cash, you can even use your "gambling problem" as a reason for picking up something new to occupy your time in a "more healthy and non-destructive way"
- Go to lunch/drinks/dinner with colleagues/friends and pay for the tab having them give you cash for their portion
- Buy extra items when shopping at a place like Costco/Sam's on debit card, return them immediately after for cash back
- Tell your friends your wife is on your ass about the money you spend eating out, and offer to fill up their gas tanks for cash back to get her off your ass
- If your company reimburses small amounts with petty cash find ways to take advantage of that
The key is keeping the amounts relatively small and the number of them high, the more transactions you have in a month the harder it is for her or anyone to make sense of the pattern. If your gas charges per month suddenly double that's obvious, but if once a month you fill a friend's tank it's likely to go unnoticed. The same for everything else.
Sorry I can't think of anything else at the moment with my Scotch addled brain
Anyway, good luck!