If I am asked to watch a machine by a person I don't know, I will flat
out politely tell them "No". I refuse to take responsibility for your
machine and property. Remember, you are taking responsibility for
their property if you say "Yes". I further tell them that any casino
personal(attendants, cleaning people, sometimes security) are usually
more that happy to watch their machine for them. Likewise it would be
stupid for me to trust someone I don't know with my property.
If I know the person, I'll watch the machine by placing my foot on the
bottom of their chair. I will only watch the machine directly next to
me on either side.
Now that being said if I say no and they still leave with money in the
machine, I will make sure no one cashes it out. If someone insists on
taking the machine with money in it, I will yell for security. Since
I didn't take responsibility for their property, yelling for security
is all I need to perform for due diligence. A player's card only is
irrelevant because people leave their cards all the time.