Let pets 'smell' invisible creatures/players.
I don’t play a ranger, and I know very little about how stealth works in this game, but this makes sense, thematically speaking.
My only gesture is that stealth tends to be a fairly brief mechanic, so the intervals for the pings would have to be pretty short. It’s not a terrible idea, but it’d be pure luck if the ping succeeded, and it’d have to be a quick reaction to utilize it. Really all this would counter is retreating folks.
So it would be less a sentry, and more a tracker.