How does cache expiration work?

[scastrianni]

I am using OPDK.  I created an environment cache with an expiration of 600 seconds.  I assume that is the default expiration of any item I put inside the cache if I don't specify an expiration on the item when I put it inside the cache?  I then put an item in the cache WITHOUT specifying an expiration.  I then lookup the item from the cache AFTER 600 seconds and I do NOT get a cache miss.  Why?