I'm a lucky chap.
I took the wheels off my bike today for a bit of a clean, and fitting new brake blocks. I deflated the tyres to remove the wheels. The rear was not soft. Then when I re-inflated the rear I found I had a very audible puncture. Searching for the sharp dIdn't take long - I found the shaft of a drawing pin protruding about 2-3mm on the inside of the tyre.
I don't know how long that had all been holding together. My ride last weekend was a wet, hilly, dark exhausting 100 miler. The pin must have been there during that. I'm so glad to have been in my own backyard with a cup of coffee to hand while fixing it, and not at the side of a dark, wet, rainly road.
Quite how the rear tyre & tube managed to stay inflated with a pin like that in it I don't know. If I'd read it on here I'd have been a bit sceptical.