Saw a note about new Retrosheet modifiers on the Chadwick mailing list.
New modifiers as of December, 2012 /FINT – fan interference /BINT – batter interference /RINT – runner interference /UINT – umpire interference /OBS – obstruction /NDP – no double play /AP – appeal play /REV – play reviewed /PASS - runner passed another runner /BOOT - batting out of turn
Unfortunately, these are not documented on the Retrosheet site, and it makes me wonder what else is missing.