Bugbear True Seeing?

[MAGIC]

I have noticed several times that some Bugbears seem to ignore invisibility. This isn't the usual "monster moves to your last position and keeps hounding you" behavior - for example ranged bugbear enemies will continue to attack an invisible target and I did not see any concealment rolls against my char when invisible. Some bugbears even take AOO attacks against invis targets moving around them.

I didn't see any log entries for them drinking potions of seeing or the shaman casting see invis or true seeing.