Leave a heat source on and stake it out... or use a cat as a navaid.

what kind of snake is it?

+1 redic on using a heat source to find it.

also, since it's already missing, it could have already found a heat source around the house.  check under and around all electricals (tvs, receivers, fridge, oven, etc.).

furthermore, do not underestimate the simplicity of snake mentality.  i once had an escapee which i found 5 inches from the enclosure, on the floor, behind the pedestial.

other than that, trying to find a lost snake sucks.  they can go for long without food and can hide in the smallest places (and i deal with big snakes, can't even imagine where a pencil thin serpent would hide).

i bid you luck!

lightly dust flour along the walls in the room it was lost in.  it'll leave tracks in the flour if it's still in the room, and you can also dust the flour near anywhere you suspect it may be - such as in front of the cage, by the sofa, etc

+1 for the heat source

and check the absolute wierdest places you can think of.  if you do not think a snake can get into a place, check it.  guarantee you that's where it'll be!

It's a northern water snake neonate (nerodia sipedon). It was in a small cricket keeper (that had a small hole i was unaware of), while I was cleaning its cage, sitting on top of a stack of ads that I had thrown away. I'm thinking it could have possibly hid in those because it would hide between layers of news paper in it's cage. I will try the heat idea although i have heat mats and cerramic heat emmiters along opposite walls and the water heater is in the closet, but he never is around them. I've checked everywhere I think it could possibly be, under every tank stand / piece of furniture, etc, in all the shoes, inside all the cabinets and drawers for all furniture / stands. 

Depending on how long you have had the snake....Continue to provide a heat source on the"empty" tank. You can relocate the tank near the floor if you think the snake can't reach its location. I have had captive bred weeks escape and then come back to the enclosure after weeks and months. Most likely this is the only environment the juvenile snakes knows. Even a missing Crested Gecko has shown up on top of her enclosure, waiting for her feeding time. GOOD LUCK!

+1 for heat and keep up the environment it escaped from. My input is to keep looking, look in the tiniest places. The place you think to yourself, "it can't fit there or get there" is the place it will most likely be. I lost a neo MBK once and found it in the bathroom inside the empty TP tubes we recycle. Took me 5 hours before I considered looking there. It made me make my home more easy to search tho.

when i first got my balls they were in a cage that they got out of all the time one would go up in the top of my closets (according how long it had to get there) the other went staight to the cooking stove an dwould go up the back of the stove where the wires are. just a idea.  GOOD LUCK!!!

dont forget to check in drawers in its room.   I've even found a lost ball in the little slide out writing section of a desk.  Best of luck to you.

Yeah i'd try the heat source thing but good luck, i had a cornsnake escape from a tank that i thought he couldn't get out of and what do u know, now u see him now u dont lol,  he too was the size of a pencil and i searched everywhere even under the house and nothing. But good luck and i hope u find him, it sucks losing something u really like.

Check under the fridge, dryer/washer, stove...Our ball python that isover 3ft long squeezed under two different doors that I thought my corn snake would have had troubl fitting under who was also 2/3's smaller than the bp.  We found him under our dryer once, and same with the corn snake.  So the laundry area and kitchen are definitely good place to look for the little guy.  If our ball python who's probably about 1,000 grams can fit under a 1/4 door gap then your water snake could be anywhere.  Good luck to ya and your little guy.

Still no luck, I had already checked all the drawers, etc. even in boxes of mail and check anywhere warmer then room temp frequently. There is a spot directly on the other side of our bedroom door that one of my fiance's rabbits had chewed/scratched through the carpet trying to get under the door when a friend had left the bathroom door open while we were gone (at the time he lived in the bathroom with a litter box and roamed the apartment while we were home), so its also a possibility he could have gone under the carpet and be anywhere in the apartment under the carpet. I want to use a tree frog as 'bait' but I don't want to actually feed it to the snake. If i take a tree frog and rub it around inside the 'trap' (tipped over tank with heat pad and hiding spot) will the scent stay there long enough to attract the snake? Or would I have to do that multiple times a day (obviously i wouldnt to avoid the stress on frog, i don't need to lose a snake and frog). Or is there a way to keep the frog on the colder side of the tank with out it getting eaten? I was thinking a super small screen cage but need to keep humidity up for the frog so that would pose a problem, i don't really need any frog chips at the moment.