The only way you can be "garnished" is if there is a judgment out there. A debt collector, with an outstanding bill, cannot garnish your bank account or paycheck.
If they call you again, get their information and send a LETTER to them requesting proof of the debt. I know about the 30 day window, but that is for us to ASS.U.ME that it was sent to a correct address. In light of the fact that this is an old debt and has the OP's maiden name, it can very well NOT have gone to her/him.
However, legally they are only required to send the initial contact to the last known address. Not knowing what the debt is for, I can't comment on how the address was obtained.
If, say, it was a medical bill, they are only required to send the acknowledge letter to the address the OP filled out. The 30 day window begins from there. If the OP has moved a number of times since then, the 30 day window does not start all over again everytime they find a new address for her.
If the collection agency does by happenstance agree to send out a validation, you can also request a proof of mailing. This will tell you to which address it was sent.
As far as a debt collector not being able to garnish a checking account, there are some exceptions to that. SOME judges will allow for a pre-judgment garnishment. The occurances are few and far between, but typically those cases the garnishments are awarded if there is genuine intent to sue pending. Like I said though, it doesn't happen often... but it CAN happen.