1. The account number means nothing to anyone but the issuing casino's computer. Cards can not be used except by the person it was issued to, and then, only when showing proper identification.

I always erase the magnetic encoding before trading, by passing a strong magnet over the strip.

2. I have harvested thousands of cards from hundreds of casinos and never been approached by anyone from the casino. I do believe that security is watching at all times and as long as they see no threat to themselves or to the other patrons, they will leave you alone.

3. You should be discrete. I have several rules that I use to decide what to take:
A. Act like a player, and play a few coins in each machine that has a card in the slot. When you leave, take the card with you.
B. Never take a card from a machine that has a player at it or next to it. It is best to "harvest" when the casino is not too busy.
C. When taking a card from between machines, also take a coin bucket.
D. Never bother a player; security is always on the lookout for scams against thier customers.

...and, of course, it helps to bee over 6 feet tall.... It's tough for us shorties to get, or even see those cards on the tops of machines.