They scan everything now. They know where each item disappears. So it would look really odd if one carrier or one sorter had multiple mail pieces start missing, so I assume that's why they'd steam and re-seal. It looks the same on the next scan as it did on the last, so they can't pinpoint one person as the last one to touch it.
Envelopes get torn open occasionally, so that's a good cover too. It got torn open and the contents spilled out.
If you haven't signed up for USPS Informed Delivery, you should. They show you scans of what mail should be delivered each day. I've caught several missing items including a financial statement. I waited two days and reported it wasn't delivered, and the next day it came.