Setting off Bank Alarm

[03 GHIA]

So I'm pretty sure I managed to set off the alarm at my local Commbank today.

I'll start off by saying I work in retail, and on Friday someone either left their card or handed one in to my store. Usually when this happens, someone will call up to ask or come back looking for it.

As of today this still hadn't happened, so I thought the right thing to do was to drop it in to the bank that the card belongs to? as I have done before.

When my shift finished today, I took the card with me to drop it in to the bank (thinking they closed at 5pm not 4pm), so when I got there they were closed.

However, in the front part of the bank they have the ATM's. As I was in there, just as I turned around after noticing it was shut, their alarm went off.
Now I can only think that this is because the card must have been reported as lost or stolen?

Naturally I then searched a contact number for commonwealth bank to say a card was handed in. The service rep attempted to contact the owner to no avail so then cancelled the card, and I have cut it up.

Now the bit that bums me out is that the service rep couldn't tell me why the alarm went off as the cards status is private information.

So I am now thinking that I will have to go back to the bank to say I was trying to hand it in and didn't steal it? Has this happened to anyone else here, and if so what's the go?