Okay. Let's try to help.
Question: The toilet is six years old and this is a new development, right?
It seems to happen "slowly" -- give us some color on how slowly. An hour? Six hours? Does it always happen now or just sometimes?
Also, how "empty" is empty? Like basically all the way out?
And to confirm, we are talking about the bowl not the tank, right?
And nothing to your knowledge has changed in your plumbing system in the last year, say?
Assuming we have a handle on the facts, my first reaction is either (a) a crack or (b) something wicking water over the weir in the area that you can't really see. You could use a mirror or the camera on a smartphone to look up the far side of the hole and see if there's anything stuck up in there that could act as a wick -- even dental floss. (Do this with the bowl empty, of course; my phone, at least, isn't waterproof.) You also might see a crack that would be letting the water run from the bowl into the trapway and thus just down the drain.
If it's what you are calling a venting issue (i.e. another toilet siphoning the bowl), I have to think that it would tend to happen in fits and starts, not just slowly, and not consistently. But I will let the others on here comment on that.
As to whether if it were a venting issue it would affect the other toilet, that is all going to depend on where and how each is vented, but like I say this doesn't strike me immediately as a "venting" issue.