Tubes can go spontaneously.

Usually if you slowly turn the tire, you can find a puncturing object. That's the first thing to do. But ultimately, if there is a puncturing offender that you can't see from the outside, feeling the inside of the tire casing will reveal it. You can also remove your tube, reinflate it to reveal the hole position, and see where the hole corresponds to the casing. If you try to reinflate your tube and it just won't reinflate, because it has a big jagged hole, it's the tube. Put a new one on, or patch it with a good patch.

i'm not that enamored of thin-film no-glue patches. I like thick rubber glued-on patches, insofar as getting long-ridable repairs. Thin films will get you home though, 99.9% of the time. Mostly I carry two tubes. After a recent side-wall failure, too big for a boot, I'm thinkin of carrying a foldable tire, but haven't done this yet.