Leaks from the air bags sometimes only occur at a particular ride height - when you heard the hissing had you dropped the car to access height by any chance, or was it sitting at normal height ?
You don't need a garage - as already said arm yourself with a spray bottle full of soap and water and get underneath - if there IS a leak you will soon find it !
Spray everywhere, air bags - especially round the bottom where they "fold" and the top where the pipes go in, spray along the pipes too as it's not uncommon for a slightly blowing exhaust to melt them
Do the pipe connections on the valve block too as the collett O rings have a habit of leaking
Don't just forget about it or you WILL end up with a hard fault