Exactly, but if you're on the register, there's no point getting a new passport unless you need one. Once you're off the register, you can get a new passport and not have to notify anyone. Any flags or notices against your old passport number wouldn't apply any more.

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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)

Both the Passport office and DVLA have a marker on their system which alerts the PPU if there is any change - this was originally introduced to act as a deterrent after the PPUs discovered people changing their name, getting new documents and not notifying.  As it's a legal requirement to notify within 3 days of any change to passport (or driving licence if no passport and DL used as identity document) this marker should be irrelevant to anyone (hopefully all on this forum) looking to play by the rules.

No worries, I wouldn't do that now, and anyway, if I did, I would be required by law to give them the new passport number (which would then stay on their system for 10 years, long after I'm off the SOR!).

If you genuinely lose your passport and get a replacement, anyone on the SOR would be required to notify this to the PPU without being prompted to do so, and even if you're not planning on travelling. It's one of the basic requirements. No different from your passport expiring normally and getting a replacement - you have to report this on your own initiative.

If you passport expires/is lost/damaged/stolen etc. and then you don't get a new one, I can't see anything saying you have to tell them that you simply don't have a passport anymore. But the minute you get a new one, you have to tell them, even if you're not going anywhere.

Just so you are aware, I have looked into this for my DH and apparently it triggers as a concern flag for the MOSOVO team.  So please don't do anything like that before you are well and truly off the SOR!  Although I do also have to wonder, what would happen if you genuinely lost your passport (or, as I did when I was 19, manage to put it through the washing machine and ruin it!!)

My understanding from various documents is that the SIS 36 alerts are simply that - a flag with no other detail. The border control at the EU country doesn't know what they alert was issued, all they know is that the UK police want them to make discrete checks and confirm where and when you entered/left a country and if you are travelling with anyone else. related, it could be SOR, money laundering, drugs, anything really. My personal experience (I travel to the EU at least 10 times a year) varies considerably - if the border agent has seen an alert before it's always a quick process usually with one question 'how long are you here for'. When they haven't, they take 5 minutes reading all the instructions on their screen, ask a colleague, call a friend and eventually let you through. I've had my passport photocopied once (in Germany). Only time pulled into a room was French police at Eurostar St Pancras - the guard is a loud voice said 'why are the british police interested in you' before taking me to a room. Had to wait until a british border control liason guy came in after 15 minutes - he apologised, called my PPU to check no issues travelling and I was on my way. I called the PPU when i came back and asked if anything had changed as never had a problem at Eurostar before - they shrugged and said it occasionally happens but they didn't know why.

There are other threads on the forum where people have confirmed that once they are off the SOR, their passport no longer flags up.

Good thinking, as a new passport should have a different number. A basic check might be helpful, or if they are as suspicious as our law enforcement, they might wonder why you would take that document with you.

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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)

This is interesting seeing as the vast majority of EU countries do not have an SOR.

If, when I come off the SOR, I am still subjected to detailed checks due to my passport obviously raising a flag when arriving at foreign passport control, I will probably get a new passport, even if there are several years before the current one expires. Maybe I'll also travel with a basic DBS check on myself to show no criminal record.

When you notify your passport number to the police, they then put that number onto the Schengen system, with a note that you are on the SOR. That is part of their standard procedures. The relevant question is do they remove that information once you come off the SOR? In fact, are they able to remove the information?

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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)

My Belgian experience was similar although I wasn’t asked for details. The guy told me “you are flagged on our system, we don’t know why but we have to carry out some things”. I pointed out that I know I’m flagged and it’s nothing new, being taken into a back room was. He just photocopied our passports and asked vague questions about where I’d been.

The experience wasn’t horrendous. I just wouldn’t want to experience it with others who know nothing of my conviction. Same goes for leaving France & Germany where the border agent picked up the phone and read out my full name to someone in front of the queue.