“Absolute” is a big word. Do we have absolute certainty we see everything that sails?
First of all, we know there won't be ships where the ice is, so that limits the area. Most of the detection is through AIS, the identification system for ships, which commercial ships need to have.
I don't think we have 100% awareness of everything that sails in the Arctic during the time when they can sail. There's a period of time when they can't sail at all, so that's easy. It's time-dependent.
I think the biggest problem is probably under the surface. You know what you know, and to know it, you need to have persistent surveillance. If you have a physical presence to observe it once a month, there are 28 days between the passes you do.
As far as the space systems are concerned, at the pole, it's still limited. It's not like a geosynchronous satellite that can look at an area all the time. They're passes that go up, and it depends on the rate of passage, so I don't think we have absolute awareness.