Do the authorities impound the aircraft owned by an airline if someone is found carrying illegal items onboard?
Yes and they also impound boats and lorrys as well.
And it can take months to get them back, if you get them back at all. Depends if they can link it to the owner or if its the operator thats been naughty. Even if its not the owner its a huge legal fight to get it back.
I wouldn't have thought its to bad in the EU but outside its a bit wildwest. The amount of hulks sitting at airports around the world rotting away is quite staggering.
Mind you the same happens in the EU as well. The Metro sitting in IOM after the cork crash being an example.