Last year I had a slow puncture, I was due to drive to Buckingham and thought my tyre looked a bit flat. Did it up and then tested it after 2 hours. It'd lost 5 ppi. I couldn't find anything puncture like so I drove to a tyre place. This is what was leaking... Slowly!
Tyre gash by
Chris_Moody, on Flickr
I think I was pretty lucky to spot it. A few days earlier I'd driven over the remains of a car bumper, pretty sure that was the culprit.