I know I've had my character somehow appear to use a stimpak in conversation, don't know if they are really using it or if it's a visual bug only, I'll have to watch out for it though (on PC here however)
Pretty sure it's just visual. I've had the VATS stimpak thing happen a LOT, but it never seems to leave my inventory or actually show that i used one in my HP meter. Still kinda annoying when that fake stim use ends up with me dying because I didn't actually fire a shot like I intended.