The other day, I got a push notification on my phone regarding a transaction that was denied due to "low account balance".
The transaction was for $326.00 and my account balance was about $90.00.
I immediately got onto my account and locked the card knowing that I did not make that transaction.

My debit card is a "contactless" card so I started thinking about where I used it and how it became compromised.
The unauthorized transaction was in a Walmart Grocery in Bentonville, AR.
I live in New York City.

I used it three times as a "credit" instead of a "debit", all on the same day.
One was in a restaurant that I had lunch in (first time there).
Then twice I used it to pay for my subway ride.
All the other times it was a debit charge and even if it was contactless, I entered a pin.

So I am thinking that it was compromised as a credit and the "unauthorized" transaction was also a credit.

Thoughts and/or advice?

I don't know enough about this to speculate as to exactly how this happened, but I would definitely report it to the issuer of the debit card if you haven't already (maybe your having locked the card is basically the same thing).

Definitely one of those transactions was harvested and an attempt was made to use your card. Absolutely report this to the issuer of the card, so they can issue you a new one.

It happens to us about once a year and have never been able to trace exactly where it originated from. Fortunately, a quick call reporting the charge and they will cancel the charge and get you a new card within 48 hours. Sorry that happened to you and important to always check your credit card statement!

I've had this happen with my debit card. It actually happened twice one summer when I was working downtown and our bank traced it to a debit machine in a convenience store where someone was swapping out the portable debit machine and then swapping it back, probably an employee.

My guess is that it may have been an employee in the restaurant (there were cases of people having some sort of reader in a pocket or under clothing so that a card could be read simply by swiping it against them), but I am also aware of devices that can take card details simply by getting close, so perhaps someone on the subway watching you use it to pay the fare: as has been said, that sort of risk is now a part of everyday life but usually salvageable if reported promptly.

Thanks for the responses.
Unfortunately, this seems to be a part of our life now.

So, there really is no way to tell.
My debits were all in major department or grocery stores - I suspect it was a credit.
Looking at the card, all you need is to see one side since all the needed information is there.
On my non-contactless card, the CVV number is on the back while the card number is on the front.

I have a Discover card which I seldom use, but keep for a backup. I used it to make a purchase at our local produce stand and by the time I got home, there was an attempt to use the card somewhere else and Discover alerted me with an email. I immediately reported to the produce stand and told them although I knew it wasn't any of their employees, their card processing company definitely had a problem. I think they use Four Square, so they reported it to Four Square. Discover cancelled the card and send me a new one. Most of the major credit card companies are quite good at spotting something out of the ordinary in your card use.

A few years ago, after having done several other errands, my last stop was to by groceries where, all of a sudden, my card was rejected. Fortunately, my bank had a location nearby where I was able to get the needed cash to make the purchase. By the time I got home, the card company called me to assure that my card had been compromised and a new one was on its way. The theft happened at a hardware store, and soon the scam was local news. I happen to use a VISA issued by my bank, and they are also good at keeping on top of 'unusual' purchase amounts. My ex and I had DIscover which does the same thing.

I agree with BET to always check your bank statement. I check mine via the bank's website which provides vendor info for each transaction so I can contact them directly if there's a problem.

I've always found it moderately fascinating that, every time someone comes up with a new way to protect something, someone else comes up with a way to thwart it. It's a game for thieves. They don't always need the money, or whatever they stole. They just want the thrill of having pulled it off, gotten away with it, appeased someone who dared them to do it, and have no concern nor care for how that affects the other person.