Turning communications equipment off is all that's necessary to disappear from radar?

Radar can detect ships that don't cooperate with it, but not identify them.

Ships are supposed to actively cooperate with a system called AIS (which is like aviation's ADS-B).

https://en.wikipedia.org/wiki/Automatic_identification_syste...

If they use AIS as required, they will be identified by receivers that they pass nearby.

Surely there's a short list of usual suspects.

Process of elimination. List of all known ships (tankers). Rule out ships accounted for (alibis). Investigate the handful of rouges.

The hard limit on naval radar is the horizon which is determined by the height of the mast it's mounted on so much of the sea's surface at any given time isn't actually covered by radar. Even when a ship happens to be on radar there's no indication of what ship it is without a transponder like AIS.

Think of the air traffic controller screens in Movies or TV Shows. It shows Delta 1234 from the aircraft’s transponder. Similar to ships, the ones with radars would still see a blip but there wouldn’t be an info on screen. The Automated Identification System (AIS) wouldn’t be broadcasting.

Not exactly...

turning off radios would prevent inadvertent/automatic transmissions

disabling transponders would reduce footprint on more modernized port control radars

not using radar from ownship reduces the chance of being seen

all these together would render the ship's ELINT footprint down to an old school blob on whatever radar plot

that's about as anonymous as you can get without active countermeasures and those are VERY obvious

Can you tell us more about the active countermeasures? How do they work and what makes them very obvious?

That is “stealth”. It’s obvious because it is expensive and secret tech. But also because if you look at the ship you see a ship, whereas the radar shows an albatross or something small. Transitions are also notable for radar operators that are really paying attention. A ship that turns into a goose and then back into a ship, for example.

I think you may be mixing up active jamming (which works like shining a searchlight in someone's eyes--they can't see or track you, but it's very obvious that there's someone out there) and passive stealth (which tries to reduce the emissions that the vehicle emits or reflects).

Anyone can make a jammer, but it only helps if you can evade your pursuer before they get a look at you optically; it's not much use for a lumbering oil tanker.

As zentiggr says, you can get some degree of passive stealth just by turning off your own radio/radar, but anything more than that requires a custom-engineered (and strangely-shaped) hull, propulsion, etc. that takes military-level money to design and would be extremely obvious in any port. So, again, not practical for this kind of smuggler.

From the POV of the radar stealth tech is mostly passive countermeasures. Active = jamming.

Add in transmitting a false AIS signal. Illegal but not hard to do.

Yes for active radar. As ships aren't flying, passive radars have a very small range.

Do you have source for the chemical analysis? Curious to read...

I'm not sure if data from the analysis will be made available to the large public anytime soon, but in the links below you can find more information about the protocol in use, measures taken so far and a statement from Petrobrás about the type of oil found in the region.

https://g1.globo.com/economia/noticia/2019/10/25/petrobras-d...

https://www.marinha.mil.br/manchasdeoleo/informacoes-sobre-o...

http://www.petrobras.com.br/fatos-e-dados/recolhemos-mais-de...

English tl;dr: Gas Chromatography and “Positive Ion Electron Impact Low Resolution Mass Spectrometry” allow them to compare the samples with known samples from other oil fields. Since petroleum has organic origins, it’s footprint is pretty much specific to the environment it comes from.