1. Margaret left of her own accord years ago and has somehow been accessing her own benefits all this time.
2. Margaret left of her own accord years ago and the carers have been claiming her benefits
3. Margaret died naturally years ago, her body was hidden and the carers have been claiming her benefits
4. Margaret was killed years ago her body was hidden and the carers have been claiming her benefits
5. Margaret was living, undetected, with her carers up until recently but has not been back since the police turned up.
Now.... forensics would show if she's been there recently - toothbrush, towels, laundry, bedding etc. If there is no sign of that it means the carers are lying. If they're lying it's either because they know where she is (dead) or they don't know where she is (as she left voluntarily) in either case they've been claiming her benefits fraudulently so they have good reason to tell fibs.
Personally I reckon 3 or 4. I find it hard to imagine Margaret has gone undetected whether it be a few weeks or several years. And I think those carers had the story ready - "if the police show up say she's just gone out, otherwise we'll get done for benefit fraud". They'd have been better saying she left after an argument a couple of months ago and they'd kept her benefits as they expected her back any day now.