Losing a credit card due to automatic user behavior

> it would certainly help to show an explicit note that attracts the user’s attention and gives him or her clear instructions about how to act next.

Signs do not work.

For an examples of signs that don't work look at any large building that has fire doors. These doors (at least, in the UK) are marked with a short, clear, unambiguous, mandatory instruction:

"Fire door. Keep closed."

The consequences of not following this instruction are, in the very rare occurrence of fire, increased chance of death.

But you'll often see these doors left open, sometimes propped or wedged open, or blocked.

signs are mostly not useful, and may be harmful. It's probably best to reduce the number of signs people get.