Brûlée has been missing for a week

Brûlée is 16 or 17 years old. He is tall, very long and very lean. I adopted him in 1999. The vet said he was around 4 at the time. He'd been found wandering around a super market parking lot. He kept Maahing and reaching through the cage door trying to snag anyone who walked by to love and pet him. An Animal Services volunteer kept changing the euthanasia date hoping someone would adopt him, and persuaded me to take him. He's been an interesting and wonderful cat. Neurotic and needy, quirky and I would call him God's way of teaching me patience. He'd Maaah so loudly and so often that I converted a classical tune that I would sing and his MAAHS would fit in as part of. Until recently, he wanted / had lay between my knees if I was sitting in bed, on my shoulder and sometimes on my face if I was asleep. I thought his recently less dramatic behavior was him giving up after I forced him to stay inside when he desperately wanted out combined with all the chaos next door with new owners, new roof, city tree trimmers in our yards for 4 days, renovations with lots of people he didn't know. But I realized he might be ill. I'd planned to take him to the vet last Tuesday. I last saw him Sunday night. I had to go looking, but not necessarily unusual. He appeared from the bushes up the street, not unusual, but rather than let me get near to herd him toward home he ambled into the neighbor's back yard. I figured he'd be home in a few minutes. Hour at most. I thought I'd let the window open enough for him to push it open if he showed up while I was gone on Monday. I got home after dark and discovered I'd closed it too tight. I called and called and walked up and down the street. My neighbor insists he saw Brulee Monday headed home after I'd been out. Somehow he disappeared in that 50 or 75'. Did the possum I discovered eating the food I'd put on the window sill for Brulee frighten him off. Did someone manage to catch him? (I find that hard to believe). Did another wild animal get him? Was he scared so far away he can't find his way home? Did he wander away to die? I don't know. I've walked up and down the street dozens of times calling. Driving the neighborhood. Talked to anyone who was out, but no one has seen him. My neighbors, who all know him, can't figure it out either. I've been watching the Animals Services Strays list. I've listed him on their Lost & Found. I've called or dropped flyers to nearby vets. I have new flyers to put in mail boxes. I don't know what else to do. I feel so guilty not realizing he might have been ill and needed to see a vet sooner than I realized. I should have tried to get in Saturday morning rather than think it could wait until Tuesday. Have I done enough to try to find him? I just know IF he could come home he would have before now. Now he's gone. I don't know where. I don't know why.

My heart is breaking.