A rather fundamental rule of customary international law is that state officials are prohibited from exercising their powers on the territories of foreign states. This rule is often referred to as "the prohibition on extraterritorial enforcement jurisdiction," and it has good policy reasons behind it. Particularly with matters relating to criminal law and national security, states guard their sovereignty very closely and wish to maintain control over everything that could be viewed as an "enforcement" activity on their territories. Actions by foreign state agents are likely to be viewed as illegitimate by citizens and residents of the target state, not to mention that local procedural/human rights protections are undermined. Extraditions are sometimes scotched because of breaches of this rule. World War I started, in part, because one state insisted on an entitlement to investigate a crime on the territory of another state.
There are lots of good reasons for this rule. Which is why it's odd to see Turkey announcing boldly to the world that its intelligence agency has abducted 80 Turkish nationals from foreign states (all of whom are wanted on what can only be described as politically-motivated charges) and intends to keep on doing so. Cross-border abduction is probably the most well-known example of an egregious breach of the rule; the most famous instance is still the illegal abduction of Adolf Eichmann from Argentina by Israel, which caused such heavy international tension that the UN Security Council had to become involved.
It is also increasingly well-documented that the government of China sends agents all over the world to engage in various kinds of "enforcement" activities against its citizens living abroad. There are many Chinese expats living in Canada and they are the frequent targets of this kind of activity. And do I even need to mention the poisoning of former Russian spies in the UK?
This kind of low-level corrosion of respect for territorial sovereignty is, shall we say, not a good thing. For more, you can read about this in Chapters 2 and 9 of our book.