There is a separate tab for SIM cards within the product. Removing the device from the product will not remove the SIM card from the product, so it still could be there.
If it’s not there either, you should submit the ICCID of the SIM in a support ticket and they can track it down.
Update: I found it in your product. To get it be not in your product you should:
- Copy the ICCID of the SIM card
- Release Ownership of the SIM from the Products - your product - SIM cards tab
- Go to https://setup.particle.io and use the Activate SIM Card option to make it a developer SIM again