I agree with Mark. I had the experience of leaving a fuel cap off one time, in the only tank with fuel in it. This was in a Cessna 180, and the tank with fuel only had about 18 gallons in it for a 20 minute flight. I did not realize that the cap was off, hanging by the chain, until preparing to re-fuel at my destination! Nice stain on the wing too. Remember the articles called “I Learned About Flying From That”?
I think that the fuel siphoning eventually reduces or stops when the fuel level is quite low, but that takes time. In a Bonanza, if fuel pressure is affected, there is always the electric fuel pump.
Ian Hollingsworth
DER Flight Test Pilot