Even though the seal is only attached with a magnet, it is apparently quite important. So important, in fact, it caused an immediate search on the side of the Interstate by the Secret Service. The Secret Service were unfortunately unable to procure the seal searching in the dark.
One would think that if the seal was so important it's disappearance would mandate such an extensive hunt, someone would go to great lengths to make sure it didn't fall off often, right? Apparently not, "It happens periodically," said Secret Service spokesman Ed Donovan to CNN. "They're only magnetic. They're not put on any special way."
So there you have it. If the President is set to be passing through your town on a particularly windy day, you might be able to pick yourself up a souvenir.
References
1.'Cops track down...' view