It’s very upsetting to lose one. Unfortunately, they are really good at escaping. First, although I know you have said that you’ve checked the tank thoroughly, if you haven’t done so already, remove all the bedding and check it carefully. A young snake can curl up into an incredibly tiny area and might lie very small and still hoping the “predator” (your hand) overlooks it. By the same token, pick up each bit of decor in the tank and carefully check every possible crevice in the decor. You quotient be the first to find a snake hidden inside something you didn’t realize it could get into. A 2-L bottle trap or two can be helpful in capturing ones in the lam.
Here is a thread with other ideas.
Do keep an eye on the cats. Sometimes they’ll find the missing snake. Best of luck.