I sent a spy and told me that there aren't any troops or ships
Ok, I can pillage the city. But...
There is something hidden there and it's not by occupation
So I sent a ship and look what I've found
The espionage report couldn't find any Tender ship. But it was there
It could be that tender ship was scattered when you spied the town - so it would not show in the spy report, but by the time you send troops it reorganized itself
The espionage report couldn't find any Tender ship.
Yes that's a Bug, you can't spy Tenders right now.
See how there's a "-" for all Units except for Tenders where there is just an empty spot?
If there were no Tenders there would be the text "No units available" as it is for Land-Units (just spied with my own account and there's indeed the text if there are also 0 Tenders), so you technically know there are Tenders in your case, but they're just not shown because of the Bug.
The issue is reported ( 99378 )
Display MoreTested today again
I made the espionage 1rst (No tender at all)
and then I sent a ship
As Tendo says, in the report is missing the numerical string "-" and it shows nothing.
I think the problem is what MaestrCZ shows here
I'm going through the same problem.
it is a bug, but easilly spotted. all the ships have the - while the last ship on the screenshot doesnt have the -. that means there are ships there figured it out the hard way
yeah, spies report, there are no tenders.. even no hyphen, but in the battle they are present. All other ships are shown correctly
it seems that spying tenders are still buggy 
Yes, they're still having problems.
But to alleviate this, when there are no such risks—under the boats.
It's a sign that there really is nothing there.
So, not really boats.
So, there's the repairman there waiting for you to dock at the port.
Thank you @GordiNZinN, i got it now 
Number of support vessels not shown on spy missions
As shown in the picture, during every spy mission I perform as a garrison spy, the number of support ships/tender is never shown.
