No idea why there's a blockade

[Architekt]

I got a warning saying there's a blockade going on between my 2 cities with a trade route. Looking at the economy scan it shows 2 segments with a dashed line: the map legend says that those are the blockaded segments. OK that's fine but I don't see any ships in any of the systems along that route. None are guarding (I've seen the guarding/blockade action before and it puts these little teeth or whatever around the system). There's no ships at all in fact on any system along the entire route, not just the part where the dashed line is.

So what's the deal? Is there like a cloaked ship doing it? How can I tell who is blockading? Like I said, I can't see anything at all that would indicate that someone is blocking the path.

Edit: Here's some screenshots to make it clearer (I'm red):
https://i.lensdump.com/i/AXhqcv.png
https://i.lensdump.com/i/AXhdH2.png

[Star Paladin]

Unless the new patch changed things, you will get those alerts, ad naseum, no matter what you do or what settings you change. If there is an enemy or neutral ship anywhere, in any system, that you have a trade route going through, congratulations, you get an alert.

[Grandma Helldiver]

I think these trip when an enemy passes through one of your systems and stops for a turn.