In the future you should put him in a rubbermaid with a locking lid to soak. That way its hard for it to get out but you can also put it where you are going to be to keep an eye on it. Dont forget to drill holes. Lowes has a large selection at good prices.

I keep a container for each snake so I always have a place to put them when needed, like major tank cleaning or soaking.

Make sure you check the under side of the counter where your sink sits. It could be perched in the pipes and hoses going to the sink.

I dont think a snake that size could fit throught the gap you described.