Accidental Appeal?

[Joe Kupka]

This play was discussed several months ago, & I need the the rule that was cited:
R1, no outs, batter singles to deep right. R1 misses 2nd & proceeeds to 3rd. While taking the cutoff, F4 inadvertantly brushes 2nd base. The ruling on this board (don't remember whose) was: R1 called out immediately, no need for appeal because it was a force. However, if it was the BR that missed 2nd, an unmistakeable appeal would be required.
I'd love to bring this up at an upcoming tournament (there's always great discussion under the umpire tent), but I want to back it up with the rule book.