He coughed. Someone finding him in the crevice had been the last thing Owen expected, and the startle was enough for him to choke on the heated smoke, sending him into a small fit. The coughs themselves weren't bad, but the triggering memory—lack of breath, horrible pain in his side, the smell of blood and dirt and gunfire—caught him off-guard and he continued to gasp even after the smoke had been expelled.

For what it was worth, he tried to wave Aziraphale off as if it were nothing. Go back to your routine, nothing to see here, just a man trying to remember how lungs worked and not recall the instant of his own death. Completely normal happenstance!