Thanks for all the replies. It seems as though you wouldn't be breaking any laws by doing this although they may not be very happy if they found out. Has anyone know anyone that's done this or done it themselves and what was the outcome ?

Context would be important too. If you told the PPU you were going to Turkey for 90 days and once there you hopped on a connecting flight to Thailand, that would look suspicious. But if you went to the south of Spain and whilst there, you took a day trip to Morocco on the spur of the moment, that might be overlooked.

If you're found to have sneaked into a Section 172 country, that's likely to be a major red flag for the PPU team.  At a minimum once you're back in the UK there are going to be lot more home visits - hopefully your neighbours aren't the curious type !!

Two points to mention that haven't come up so far. In relation to what punter99 said about notification, once you are outside the UK, your notification requirements aren't enforceable. So, unless you get a stamp in your passport and the police inspect it, how would they know? Secondly, if you were in a foreign country and decided to travel on to a Section 172 country, if the police ever find out, they might well want to take further action of some sort.

=========================================================================================================

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)

I think it depends on what you knew and what you did not know, prior to departure. The guidance talks about notifying dates of intended stay in any country being visited, but the legislation only mentions the first country that you visit. The legislation also then vaguely talks about "any other information the offender holds" about his "movements outside the UK". 

https://www.gov.uk/government/publications/guidance-on-part-2-of-the-sexual-offences-act-2003/guidance-on-part-2-of-the-sexual-offences-act-2003-accessible-version#section-two-travel-outside-the-united-kingdom

If the person always intended to travel on to another country, once outside the UK, and did not notify that in advance, then the courts would probably view that as a breach. For example, if they booked a connecting flight in advance. But if the person decided on the spur of the moment having already arrived in the first country, that they would like to visit a second country, then there is no requirement to notify. 

If the police subsequently found out and took the person to court for a breach, then the judge would look at any available evidence. For example, did they research visiting the second country on the internet, before leaving the UK. If the CPS could prove pre existing intent, they might decide they could win the case.

If I was abroad and I was then going to visit another country I hadn't made a notification for then I would email her to tell her what I was doing. That way you have it in writing you have notified them of a change of plan for your travels. Although I don't think you are bound to do this.

If I was aware I was going to visit that country before I travelled I would make sure it was notified at the station.

The regulation say you have to notify them of point of arrival in each country you will visit. It does say to notify them of any changes prior to departure but does not stipulate you have to do it after you leave the UK.

I just asked them. And that was their answer. In reality, I never travelled to places which were not notified. But since you have to notify in person at a police station I don't know how you can do that while you are in Turkey (or wherever). At the end of the day it is up to you and the relationship you have with your PPU. I had/have no problems in telling them, even with an email ,where I am and where I am going to, but if they said ' you don't have to' means that since you cannot go in person to a UK police station, you can go wherever you like. And I guess that if they ever find out, they can't do anything about it. I am not a lawyer and this is just my opinion

I am just telling you what they said to me when I asked the same question to them. I am aware that it might depend on the PPU . In any case, I have always been honest with them and told them always about my whereabouts (even when I was abroad - via email). That was MY experience and I reported what I was told. 

I am no lawyer, but for questions like this, rather than relying on the opinion of a random PPU officer,  I would turn to the relevant legislation. 

The Sexual Offences Act 2003 (Travel Notification Requirements) Regulations 2004 (here) gives the baseline requirements for travel notification.  You may also be subject to one or more Court Order (SHPO etc.) which may impose further stipulations on you.  To repeat, I am not a lawyer so this is not legal advice just my personal opinion.

I always ensure I am 100% compliant with the law as written, then use this as the basis for filling out the form.  If the PPU / central travel unit don't like what I've put, not that they've objected so far, then I can point at the relevant section of the Regulations to support my case.

What about when you arrive back in the UK so you need to inform them about the extra countries you've been to ?
So you're saying you could tour Europe and as long as you give them your departure airport and arrive back on the date and to the airport you told them about you don't need to give any extra information at all ?