I am surprised at how often I hear it raised as an objection that if a large animal swam up the River Ness through the metropolis of Inverness it should attract some attention at least.
The best reply to that is an analogous case of a large released captive crocodylian (called an alligator by the press) into the White River some twenty years ago. White River runs through the city of Indianapolis and the city of Indianapolis is about three times the size of Inverness. According to Wikipedia, Inverness had an estimated population in 2008 of 56,660 while Indianapolis had an population estimate of 1,774,665 in 2007.
And Literally NOBODY in Indianapolis was aware there was an 'alligator' in the White River at the time. There were not even any rumours circulating about it: our press services picked up the story from the National Press services. Periodically there are large sea animals (whales) that go up the rivers in the middle of big cities like San Francisco or New York City. Unless somebody lavishes a lot of press coverage on the event or writes a book nobody pays any attention. The tendency to disbelief is THAT strong.
So actually it is not difficult to believe that it happens every so often Nessie goes swimming down the centre of Inverness (usually in choppy floodwaters and at night, according to some of the witness reports when something actually was reported) and nobody even bothers to go take a look. As a society, we are THAT apathetic.
[Many instances of errant crocodylians are listed in L. Coleman's Mysterious America among other sources. The instance I mentioned in Indianapolis was after the publication of that book. I subsequently read about it in FATE magazine; the story had not been carried locally. I live in Indianapolis.]