A limited number of Prius' get stickers that allow them into HOV lanes even if the driver has no passengers. A friend of mine pointed out that these stickers add substantial value in the secondary market. A story from last spring on NPR notes:
The California legislature approved 85,000 permits that will allow lone drivers of hybrid cars to drive in carpool lanes. The permits come along with big, yellow decals.
In February, the DMV mailed out the last of the 85,000 sets of stickers. Now the only way to get a new one is to buy a car that already has the stickers.
That difference is turning up in the cost of used cars, say the people at Kelley Blue Book. The re-sale analysts have figured out that a used Prius that already has the stickers is going for $4,000 more than one that lacks them.
There have been reports of stickers being stolen off cars — but the DMV doubts this is happening. The decals are tamper-proof. If you remove them they crumble and read "VOID."