If the package was just shipped, you may see this notice on the USPS website:
“The U.S. Postal Service was electronically notified by the shipper on September 23, 2008 to expect your package for mailing. This does not indicate receipt by the USPS or the actual mailing date. Delivery status information will be provided if / when available. Information, if available, is updated every evening. Please check again later.”
Note that in some instances, the USPS also omit to scan packages so their website always reflects this message, even after the package has arrived.