lastnight i got out of work just after two, i get outside realize it's warmer than i thought and so i take a minute to reaarrange my layers(take off my sweatshirt that was under my wind breaker) and then i get on my bike to realize the rear is flat. so i walk to union square and wait for the train since i didn't feel like changing a tube at 2am. waiting for the train i inflate my rear tire to find out where the puncture is, i get it up to 90 and realize there's no puncture, i turn my bike back over, sit on it, bounce it a little, not a leak to be heard or felt. so obviously someone thought it would be funny to deflate my rear, so i go to top off the rear tire and since i've been using a stupid hand pump i break the core off of the stem. since i was already waiting for the train it wasn't the end of the world but for a breif second i thought i was going to get to leave the platform and ride home but because some jerk was being funny now i need to replace my tube...seriously don't do that. it's not cool
Some leaks take hours or days to deplete so much that they're unrideable. These are found by submerging the inflated tube in a sink to watch for a few bubbles or, on the side of the road, you can lick suspect areas to find leaks that aren't quite as slow as that.
Of course by then it's too late to know you could've inflated it & made it home okay. Occasionally you'll get annoyingly slow leaks like that, too slow to find & patch quickly, too fast to ride w/out re-inflating every few hours. Sucks when you're out of fresh tubes.