So my two theories are that if they are in the river they didn't go willingly. Either misadventure or a malign interference. If they aren't in the river than I believe their absences are voluntary.
For two 32 year old women they sure keep in contact with their parents and siblings a lot. That's what we keeping hearing; they are in constant contact with their family weekly, sometimes even more so. Since Scotland is no longer in the EU I doubt they are getting free calls. Maybe it's me, but I find that a little excessive. They appear to work minimum wage jobs so if they are socking money away to buy a property calling home all the time sounds weird to me. Wouldn't it be simpler to send emails? I have a sister in the UK and we email each other. It would cost us a fortune to be calling a couple of times a week.
What do we know about their private lives? Do they have friends, boyfriends? Do they belong to some social club? Maybe transplanted Hungarians? How well do they converse in English? They've lived there for 10 years according to some news outlets and as little as 7 in others. Some people pick up languages easily and others don't. Is English difficult for a Hungarian to learn? I worked with a woman whose husband couldn't get the hang of the English language. He was a well educated man but consistently pronounce hard C's as Ch as in cheese and and pronounce every vowel and consonants. For Cambridge, he'd say Chambridga. He couldn't keep up with the language. Being a maid you don't really need to interact with the public.
Have they orchestrated their own disappearance for their own reasons?