[phillc73]

AIS can be turned off. This does raise red flags, but it doesn't seem to stop ships doing it, about 20 million times.[1][2]

[1] https://www.wired.co.uk/article/ships-turn-off-tracking-syst...

[2] https://usa.oceana.org/publications/reports/avoiding-detecti...

[beerandt]

It's also trivial to spoof.