One unused, roundtrip standard award is now missing from my account; it is valid till May 2016. It was just there, yesterday.
Scrolling through upcoming trips, I can confirm that I did not use it.
The following message is displayed, when checking awards in my account:
There are currently no Rapid RewardsŪ awards in your account. You might have awards that are currently reserved for future travel, or you may not have earned enough credits to redeem for a rewards flight.
Now, a few minutes later, the award is back in my account.