I recently had a leak from the spoiler of my 04 WRX which the dealer solved a few weeks ago however when I got into the car yesterday I heard water draining from inside the car. I checked boot and to my horror found water in the boot sill around the spare. Could this be the sunroof leaking or where is it coming from. Anyone had a similar problem?

 

Check the seal around the tail lights. Mine had the same problem and my mrs actually found it. Depending on age of car, mines 99, the original seal around each tail light might have dosed. Take the nuts off each stud on the light from inside and with a bit of careful pressure, push the tail light straight out from its position. Dont forget to disconnect the lighting connector block. Scrape off all the old sealant from the light and clean the paint work on the car down with solvent. I used Autoglym Intensive Tar Remover.
Get a cartridge of TIGER seal from your local car parts shop. It needs a cartridge gun to apply. Pile loads of it on round the edge if the light, at least 5mm deep and put the light fitting back in. Repeat on other side and I guarentee it wont leak again. If the lights need to come out again for some reason, the Tiger seal is a flexible rubber sealant the doesnt go hard.
While you are in the boot, lift the carpet up in check with either your fingers or a torch, down the inside of the wings, there is a pocket on either side that can collect water if the drains are blocked. Empty out and dry with hair drier.

Have fun