FWIW, I flew PBI-EWR on Friday afternoon and something similar happened. I am Plat and was flying with my wife. Took myself off the upgrade list because when I called, there was only one seat available and didn't want to screw someone over by getting upgraded and then switching with someone in economy.

When I got to the check-in counter, I was offered an upgrade for $150. I turned it down, obviously, but checked my PDA to see what the standby list looked like.

It was about 8 people deep with 13 booked and 10 checked in (about 45 mins before flight).

I ran to the gate, put myself and my wife on the standby list and waited. I was first (and my wife second) for awhile, but eventually, with two seats open with about five minutes before they cleared the list, someone bought a full-fare ticket and got an upgrade. I declined mine and so did my wife.

The upshot is that plenty of people were on that upgrade list when they were offering to sell upgrades. Whether or not they were silver is unknown.