There's really no way of telling when that snake was in your house, because as you said it's been sittin in water. A snake skin could fully dry out and get put in water and rehydrate. It's from a garter, non poisonous, so i really wouldnt worry. being that cold, he's brumating somewhere, most likely outside. you may want to check though under any close by places that are very close to the ground - if you have a washer or dryer closeby, look under there... under boxes, etc. Look where you think theres no chance of it being, because thats where they go.
as far as the length, i'd say maybe 2 years?
your cat wouldnt be acting funny unless it sees the snake. they don't have a 6th sense to "know" if a snake is close by.
I dont really think theres any way to tell if its still around or not.. but i wouldnt lose sleep over it. If your wife or kids could be fretting about it and losing sleep, tell them that it's out of the house. not gonna hurt them even if its in the house, and they won't know that you dont know if its around or not.
-----
-Chris
The reason mainstream thought is thought of as a stream is because it's so shallow. -George Carlin
A fool doesn't learn. A smart man learns from his mistakes. A wise man learns from the mistakes of others. Which one are you?
My Website
Rat/Corn snake care sheet I wrote
Current snakes:
0.1 Licorice Stick Black Rat (Lola)
1.0 Black Rat (Frankie)
1.1 Texas Bairds (Jose and Rosa)
0.1 Blue Beauty (Brunhilde)
1.0 Green Tree Python (Claudius)... coming soon