Took in a stray for six months and now he's left - why?

I took a stray neutered boy in six months ago. Approx age 2 yrs.  Bertie is the most placid, loving & playful boy and has mostly stayed inside until the better weather arrived and then sometimes might stay out over night but always came home the next day.  He was unwell at the start of the week and bearing in mind he has been to the vets before the morning after he went he was let out and has not come back since.  We know that he hangs around a lot on a new building site over the road where there are now people moved living in some parts.  I went over last night to see if i could see him and yes he was there.  I called him but he ran away from me.  At least i know that he is ok but cannot understand why he has suddenly decided to leave us when he has had such a loving, caring and safe home.  Can anyone shed some light on this because i am so worried about him and miss him terribly. Thanks.

 I'm afraid I don't have any answers for you. If it was me, I'd continue to try to catch him - take his most favourite stinky food and see if you can bribe him over. Hand out flyers and put up posters in the area you saw him too. I really hope you're able to get him. I'd seriously consider making him an indoor only cat if you do get him back - there are just too many dangers in the big wide world. If there's a chance that he's ill, it's even more important to keep him where he's safe.

Good luck