Lethargic Man (anag.)

My spare bed, until last weekend, was an air mattress. Unfortunately, now it's no longer staying inflated throughout the night, which rather stymies my ability to put people up. curious_reader says that all the airbeds she's got do this after a while, and all the reviews she's seen online mention this; but I reckon there's no smoke without a fire, so I set out to cover every part of the mattress in turn in water, and look for bubbles, as I would with a punctured bicycle tyre. (I just wish I'd googled first and learned about using soapy water, and then I wouldn't had had to mop up a flooded bathroom floor afterwards...)

Anyhow, I couldn't find the leak. So, does anybody have experience with air mattresses here who can advise me? Or is it time to start looking into an alternative means of low-volume stowable spare bed...