Beyond devastated

I have a stray cat that eventually stayed in our home after feeding it a few times. He was a very active one and would "Meow" at the front door every time he's hungry. He's been staying with us for a year and a half now.

Just three days ago, I noticed that he didn't do his usual routine, so I checked up on him and he seems weak. I thought he was sleepy, so I gave it no thought and still put food on his own bowl.

After a few hours, I checked the bowl and no food has been eaten, with my cat nowhere to be found. It has been three days and he still hasn't returned.

I have searched around Reddit and just recently discovered that a habit of cats is to hide themselves somewhere when they feel like they're dying. Reading this fact devastated me. I don't know what to do, I have searched around the neighborhood and haven't found anything yet.

I am gradually accepting that he might be gone, but I am still hopeful that someday, he might return and "Meow" by our front door again.

His name is Delta, and I can't even post his picture because I get too emotional when browsing for his pictures on my phone. I just want this heavy feeling to go away..