You cannot trade with NPCs while invisible or feigned.

[Haenir]

I've gotten this message during a two quests now, where I'm hiding as rogue and observing some part of the quest. Does it indicate that I should have received an item into my inventory (and didn't) or what?

The two quests are:

Allies and Enemies (after the Spiderkin Broodmother kills the General in Dreadlands)

Ned Timsis' quest (after Captain Algo is killed)