I have seen this about 3 times now:
after leopard install, spotlight indexed my local drive and my external usb.
As I use an mbp, I dont always have the external drive connected. I noticed, that after disconnecting the drive for a couple of hours and then reconnected it later, spotlight started re-indexing the external drive. The same thing has happened today, so this is the 3rd time this external drive has now been indexed
This never happened under tiger 
after leopard install, spotlight indexed my local drive and my external usb.
As I use an mbp, I dont always have the external drive connected. I noticed, that after disconnecting the drive for a couple of hours and then reconnected it later, spotlight started re-indexing the external drive. The same thing has happened today, so this is the 3rd time this external drive has now been indexed