RE introducing cats???

Question on cat intros. As some of you may remember, I had a foster boy for a few months named Chance. Well, we were very happy for him when he was finally adopted 2 weeks ago. The couple seemed very nice and he took to them immediately. Unfortunately something happened in their home and the couple is now getting divorced. They had 2 cats - a 2 year and this little one, Chance. I guess the woman's sister is taking the 2 year old cat for her, but will not take them both. The woman is now living with her grandparents, both of whom claim to be allergic to the cats, but after being in the house for 5 minutes to pick Chance up, I can tell it's more of just the fact that they are not cat people. There's just some people you can tell with. The grandmother was hounding me with questions as to whether this sweet little boy had made a mess upstairs in the room they had him in for less than 24 hours and the woman I was taking him from told me if it wasn't for her grandmother she'd be keeping him. Very sad. I felt terrible for her - losing a husband and now both her kitties.
Anyways, long story short - Chance is now back home with us and my question is on introductions. I am familiar with introducing new cats to one another and I also know how a cat can come home from the vets or something after a day and smell different for a while, but that goes away and they remember one another. My question is - this has been only 2 weeks. Kind of right in the middle and I'm not sure if I need to go through a complete introduction as if he is new again, or if after 2 weeks if they will remember one anothers scent. I don't think it's been long enough for even his smell to have left our home yet has it?
Thanks!
for now he's upstairs alone just to make sure

One thing you could try is putting little Chance in a carrier and leave him where the others can go up and see/smell him. If there aren't hisses and growls, they likely remember him.

You can also try the vanilla trick - put a little dab of vanilla (real vanilla works best) under the chin, between the shoulders, and at the base of the tail on the back of each cat, including Chase. That way they all smell the same.

I'll move this to Behavior for you. Someone may be able to tell you better how long cats will remember each other.