why would my hazard lights turn on when the engine is off (ford focus)?

i went to work yesterday in my van leaving my 2001 ford focus at home. when i went home for lunch the hazard lights were lit up, i had a quick play and when the engine was started they went out and everything was normal but as soon as the endine was turned off again they lit up. i had to give to get back to work and when i returned that evening my batt was completely flat. i need to replace the battery but don't know what would have caused this problem in the first place and want to get that resolved at the same time as fitting the battery. has anyone had a similar experience or any ideas on how to sort this out? cheers folks