Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help.
I'm so sorry for your loss. I know how heartbreaking it is because I have been in a similar situation.
We will never know when Tilly may have gone under the patio. Since you looked there previously, it is unlikely he was there the whole time. That leaves the possibility that he was dying and crawled under there after you looked or a person put him there after he was dead. It seems odd someone would do that, but stranger things have happened.
I know you feel helpless or that there was something you could have done differently, but in reality, it is very likely there is nothing you did or could have done to make a difference.
The most likely scenario, in my experience, is that Tilly got into some trouble, likely a toxin and came home to die - I'm sorry to be so blunt. Since he was only 2.5 years old and you found no signs of foul play/trauma a toxin is most likely.
If he died fro trauma, you would see that on the autopsy. If he died from an infectious problem, you would have seen signs before he left home. A different medical problem, like heart or kidney disease, is very unlikely given his age.
Pets are known to come home when they are sick but if they are dying they tend to isolate themselves, like under the patio.
He may have been too weak to whimper or bark which means he probably died shortly after he crawled under the patio.
As to why the other dogs didn't alert to his presence, it is possible his scent was altered due to a toxin.
The results of the autopsy usually give us a good indication as to the cause of death and I hope those results bring you some closure.
So, no, based on the information you provided, I don't see how you could have done anything differently to help Tilly.
Sometimes it helps to talk to someone, click here
for a link to a pet loss support site.
If you have other, more specific questions, or need to talk, please let me know. I"m happy to answer questions even after the autopsy results have returned.