[trevius]

I think this is the code that evasive is using to add the bonus to avoid chance:

spell_effects.cpp

			case SE_AvoidMeleeChance:
			{
#ifdef SPELL_EFFECT_SPAM
				snprintf(effect_desc, _EDLEN, "Avoid Melee Chance: +%+i%%", effect_value);
#endif
				// handled with bonuses
				break;
			}

And this may be there the bard song I mentioned is adding the bonus:

			case SE_ProcChance:
			{
#ifdef SPELL_EFFECT_SPAM
				snprintf(effect_desc, _EDLEN, "Proc Chance: +%+i%%", effect_value);
#endif
				// handled with bonuses
				break;
			}