Believe me, I am aware that this sounds like the plot of a bad novel, and I'm a bit hesitant to post in case I just get called a troll, (have namechanged) but anyway...
I split with my ex-p about 4 months ago. Reasons unimportant but it was my decision and our DC (both under 7) stayed with me. He sees them regularly but has not had them overnight since we split (has not asked to have them).
After a few early blips (threats and him showing up at the house uninvited) he has been pretty reasonable. The usual passive-aggressive small things, but nothing major.
We are not in the UK, we live on a small island. He is American and the DC have US citizenship through him. I am British and they also have UK passports. He has asked to take them to the US to see his mother this summer, and so far I have stalled him. My lawyer advises me not to let him take them, but the fact is, I cannot stop him. As the DC are American, the US will let them in without travel docs. Or he could easily apply for new US passports for them, saying the old ones have been lost/destroyed. Once they are in the US I will not get them out if he does not want them to leave. But if I refuse him 'permission' I think he is more likely to take them. He has been back and forth from the US on about 3-4 very short-notice mystery trips since we split, which is unlike him. I worry that he's setting things up to disappear with the the DC, but I don't know how I can stop him.
I suspect he has surveillance/tracking of some kind on the house and storage, or maybe in my car. e.g., my friend and I popped to storage this morning to pick up a few things and he randomly turned up 5 minutes after we got there - did not stop or collect anything, just said he stopped to 'say hi'. Storage place is in the middle of nowhere, not somewhere you'd ever just 'pass'. I wish I could find any watching/listening things but I don't know how. There are no investigator type people on the island I live on. He is ex Special Forces so very good at tactical planning and surveillance type things. Still has lots of military contacts.
I know I sound paranoid, but I am worried. Don't know how to safeguard the DC and/or to find out how he knows things he shouldn't know (where I am, things I have said etc). Please hold my hand, all my instinct is screaming that he's got something unpleasant up his sleeve.