Chances are good it was a small puncture from
concrete aggregate or something on the floor.
If it were pool chemical the entire can bottom
outside would have been corroded. (A friend
never used his pocket change, but threw it in
a plastic bucket to cash in some day.
He left me one of his buckets. Turns out it was
a rinsed-out pool chlorine tablet bucket. The silver
cleaned up OK, but the copper coins were hard
to identify - about $100 worth of pennies the banks
Plastic-bottom paint cans avoid the corrosion
problem, but if you stir paint on the cheap as I
do with a bent coat hanger chucked into a drill,
you have to be --very-- careful. :) Setting the
can on a stray screw guarantees a mess - somewhere.
BTW- A sealed can cannot pick up water (and
subsequently rust) from anywhere.
If warm humid air can get in, when it
cools, the water condenses (like dew at night).
The next day more warm, humid air gets in, etc.
If air can't get in, the only condensation possible
is from the air in the can when you sealed it. This
is a major problem with vented petroleum tanks
where feet of water a month have to drained off