If he only "just" went to Germany, it might take a while for the Border Force's entry information to be sent to his PPU ...

There is an RSO (more than downloading, served several years in prison) who went to the US on holiday without notifying. He got into the US no problem by ticking "no" on an ESTA, but was arrested on return once the PPU got the information about his arrival from UK Border Force. He was sent back to prison. The story can be found online but I'm not sure I should link it as we all feel the same way about privacy and the Google effect.

As an aside, this would suggest that unless you tell the US about your conviction, you're very unlikely to have a problem when travelling on an ESTA, but it would be best to wait until you no longer to notify under the SOR.

He long ago (maybe 3/4 years ago) went (only\just) to Germany. and then back, with no ill consequences for himself.

Moreover he did it for our benefit! To try and elucidate the actual mechanisms involved and provide us with a data point.

So we him one I guess!

The UK Passport Office system has a marker field that can be flagged by the PPU (as does the DVLA) so that they are notified automatically of any 'activity' such as ordering a new passport or changing a name. I don't know whether this marker is set for every RSO or only those where the PPU assess there is a risk of name change (in one of the police documents I read it discussed risk factors such as a job that required overseas travel, other convictions that had a fraud element,....)

The information regarding SISII can be hound at https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32007D0533 
The information stored in the database is:


Details on accessing what SISII alerts are on the UK system for you can be requested (see https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/421540/SISII_General_Information_document.pdf), and you can request disclosure from each member country individually. 

Regarding the ability to see SISII information on yourself, the information in that document (and others) and the link simply go to the ACRO website for doing a subject access request on yourself. I.e. a complete extract of your criminal record (spent and unspent). I already have one of these on myself so I know what it looks like - but this surely is different from what is seen on SISII? When you do an ACRO request, you can't see anything specifically relating to SISII, like reasons for an alert, etc. It just goes around in circles and ends up with the ACRO SAR. So I'm not sure if there is actually any way of seeing what SISII has on you.

Or do you specify in the request that you are expressly looking to view your SISII details rather than your general criminal record?

I would expect the SISII information to be a mixture of what ACRO tells you and information held locally, equivalent to the approved information disclosed on an enhanced DBS check. If you apply for copies of both sets of information, that should cover it. As with enhanced DBS checks, I doubt that the police would add any information that is not held either on the PNC or in local files.

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If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)