Intermittant leaks do happen, but they're rare. Possible causes include a faulty valve or possibly a grain of sand at the seat, a leak at the valve/tube connection that might only leak if the valve were bent one way (these tend to get worse), a puncture caused by a fine wire, small enough to self seal if the tube moves slightly in the tire.

Since you're OK for now, you have a choice. You can ride for as long as your luck holds out, or you can replace the tube, and try to find the leak to satisfy your curiosity. If you do remove the tube, fill it to about double it's thickness and hang it up to see if it holds. If it makes it 48 hours, ist's probably OK to use.