I mean the only "ghosts" we could have found were the Zurara crew. (LOL !)
A little poor to justify dozens of legends and myths talking of "ships disappearing" and others "weird heat signatures" commented in every space station bar of the bubble for years.
Or, when using old tech spending days in witch space the Thargoids living there would react as you would in seeing a sailing ship crossing the ocean. They would see it travelling from point A to point B and would often have time to intercept, follow and see what humans were up to. This meant in areas where they had a interest and concentration of population such interceptions and monitoring were a common event and many ships were lost. But crashed condas and below are not easy to find. Considering the vast number of exploration data caches we find though it's definitely thousands of ships lost.
But now they only see us for a few seconds in witch space with our current drives. We are there then gone before they can react. Now more like a supersonic jet flying across the ocean rather than a sailing ship.
They have had to relearn tactics such as in an area of space surrounded by probes watching over their fuel sources. Rather than moving to intercept our passage they now need to be on our route waiting to ambush us. With it now being hundreds of ships rather than the odd one their resources are stretched.
And maybe if they felt a sailing ship crossing their territory was an intrusion 1000 low flying supersonic jets is even more of one and they have been trying to warn us off. Maybe they decided to put up with the sailing ships as necessary after the last war but think 1000 jets is too much.
It's easy enough to justify the mystery, the time since the previous war, lost ships and rumours about certain areas.
I would hope FDev have a logical justification for their absence, ramping up of events, reactions to humans and the tales told across the flickering embers of a midnight fire, wherever hardened space dogs gather to drink fermented vegetable products and compete in tales of blood-chilling terror.
The fact we don't have all that yet doesn't mean it doesn't exist or that it does, but I hope FDev do have it and it's part of the story narrative that will come out over time. If not I will be disappointed and fill in the gaps myself in my own imagination for my own enjoyment of the building threat. For now though, I'll give them the benefit of the doubt that they do and that things already placed in the game or more likely to be placed in the game will reveal this over time.