Today it was a problem. It appears that a slowly deflating rear tyre does... interesting things to the handling. The rear end was swinging from side to side in a way that would not be good for anyone who seasick Of course this is why I carry a hand pump at all times in the hydration pack and that got me home. I checked the tyre and pumped back to 70psi using the floor pump when I got home but couldn't see anything that might be causing a leak.
Tomorrow I'll check the pressure again and look at pulling the wheel for a closer check if there has been a pressure drop.