Q:

Found our indoor cat after two months.  Why do our other cats hiss when they see him?  They were best of buddies for three years

Proud to say after two months and hundreds of flyers given out and posters and trail cameras set, someone spotted our cat 4km away and we got her back yesterday.  We took her to the animal hospital and she is doing okay so far.  She is currently locked alone in a room and we are supervising her. 
However, I noticed whenever the other cats see her in the last day (there is a transparent cat door in that room which is locked now) they start hissing at her and changing their attitude towards her.  My cat that was missing however seems to still be happy and wants to play with her old friends again.  Why exactly are my other cats so aggressive towards her?  They were best of friends since they were young for over 3 years. 

A:

Cats identify each other by scent a lot more than by sight, and after a cat has been gone for two months he will have picked up a different smell that the other cats do not know. 
One can often see this if one of the cats has been at the vet; when the cat comes back it will take some time before the other cats know him/her because he/she smells different. 
Take it slow when you re-introduce the cats so they get time to recognize each other and each other's smell.  A little hissing is often just a warning to the other cat and is a sign that one cat is unsure about the other. 
In your case it is best to provide your cats some safe spots so they can make a quick retreat if there is any problem. 
You can take one of your used t-shirts (unwashed) and put this where the cat that was lost sleeps, then move this t-shirt to where the other cats sleep the day after.  This is to transfer the smell of the cats and you to make the re-introduction a bit easier for the cats. 
Depending on how the cats react to the t-shirt, you might have to move the t-shirt between the cats several times.