Legally, all pets need to be checked at least once in every 24 hour period. Many animals would normally be ok if left for longer periods with access to food and water, BUT, 24hrs is deemed to be the longest period it is acceptable for an injured/ill animal to be left without attention.
If there is anyone, whoever has agreed to look after these cats in the owner's absence is legally responsible for them (and any vet bills unless previously arranged!). As far as I'm aware, the RSPCA would use the tape across the door method to establish whether someone was entering the house or not. So in the case upthread where people were going in morning and evening, I would be asking the RSPCA for what evidence they had that the animals were not being attended to. It's a very sad tale but I'm sure this would have been a fairly isolated case.
Whether or not a cat is seen in during a 24hr period could be an issue, but it's about building up a picture over several days plus any knowledge of normal behaviour. It might be clear that food/drink is being taken, in which case it might be reasonable to assume the animal is ok. If food is left uneaten and the cat hasn't been seen for several days, then you likely have a missing animal and reasonable steps should be taken to locate - searching/posters up etc.
So OP, I would have a think about whether it's possible for anyone else to be checking on the cats, but I'm afraid there are a lot of people out there who think it is ok for animals to be left with automatic feeders, and checked only every couple of days or so. (And agree the rabbit on the trampoline isn't exactly building a great picture.)
Disclaimer: I used to work in a vets, but many moons ago!