CAVEAT: I'm not a merchant mariner but rather a naval officer. My sense is that there are huge operational differences between the two communities. So my impressions may not be very useful. But I'll take a crack at it.
Depends on what you mean by "planned route". My at-sea experience has been that the courses of merchant shipping are usually pretty easy to understand. They tend to stick to known shipping/transit lanes in the open ocean (international waters). They do this, not because they are required to, but simply because these are proven, fast, and easy routes between destinations where their cargoes need to go.
Shipping lanes in open ocean seem ~10-20 nm "wide". It's important to understand that there are not really formal "shipping lanes" on the nautical charts I'm familiar with. (Except near ports and in territorial waters...there many countries establish traffic separation schemes to avoid conjestion. These show up on charts).
I'd guess that merchant ships have a planned route on file with the shipping company that owns them though. That is coupled with satellite phones and Automated Identification System (AIS) that report ship's position with an accuracy on the order of tens of meters every so often. I'd imagine that Big Ship's, Ltd. Operations Center would notice pretty quickly if a vessel they thought was going to Hong Kong instead headed for Tokyo. They'd probably call the ship in question on satellite phone. If they didn't get a response they'd probably assume some kind of casualty/hijacking/problem.
Now, you're asking in the mission design forum for a game about running warships, can I assume you were thinking of having someone hijack a ship and the player intercept it? This has happened frequently off
the Somali coast in recent years and might make a very interesting mission. The questions there become:
1. Does the crew get a chance to get a mayday call out?
2. Does anyone hear it?
3. If they hear it, do they pass it on to authorities?
4. How long does it take for the call to reach people with actual naval assets that can do something about the problem?
On those questions, anything I say would be a swag: "it would depend...". If this is your motive for asking, I'd look into the individual incidents at the website above and see what how long the incidents took to play out in RL.
R/
feld