'Tis true - happened when Paul was in LA and Derek Taylor took him along to see Brian recording "Vega Tables", 4/10/67 at Armin Steiner's Sound Recorders. The general description of the recording of "Smokey" is "impromptu".

This is the same session where Paul played Brian (depending on whom you believe) either an acetate of "A Day In The Life", or "She's Leaving Home" on the studio piano. The concensus is the latter.