Shopkeepers and invisibility
The message "you feel observed" is displayed when PC is not in the shopkeeper's line of sight.
If the PC is invisible and is on a dungeon level with a shopkeeper in it that has already noticed the PC, the message "You feel observed" is displayed at random intervals even if the PC is not in the shopkeeper's line of sight or inside the shop.
If needed, I can provide a screenshot as this PC is still alive and kicking.