What is "Verifying last 288 blocks at level 3"? Upon every start of bitcoind, I see the following in de debug log:

Verifying last 288 blocks at level 3


come up. At this point, no new blocks have been downloaded and the block chain was 100% up to date when I last closed bitcoin. Bitcoind does not do anything before it has 'verified' these 288 blocks (the server does not answer until this phase has passed).


Why do the last blocks need to be verified if nothing new has been added since shutdown?
Does this verification make any changes to any files? Does it save anything to the blocks / chainstate / database directories?
It is to verify the block database has not been damaged or tampered with. It does not modify the databases in any way.