Having learned a bit about 'disappearances" thanks to some famous cases, I really think this is unlikely. Especially in light of how UK police have significantly improved their media control since a couple of cases I won't mention!
The police develop competing theories of the case, and then as they investigate evidence will tend to support one theory - showing them where the truth lies.
In this case the theories were obvious from the start
1. A fight
2. An affair
3. Suicide
4. Stranger murder
5. IS whot dunnit
Obviously fears over Helen heightened swiftly when she was not using her phone or bank accounts - meaning 3 , 4 and 5 become more likely.
3 swiftly becomes unlikely when no body turns up.
So I think we can see that IS would have been one of the main theories of the case, if not the main theory of the case from very early in the investigation.