Help system sleeping? - not everytime

[Silly Jilly]

For the price I think this phone is great. But, sometimes when I wake it up, the battery has been exhausted while it is supposed to be idle. I only have wi-fi, data transfer, gps etc. on when I need it.
normally you need to press the power button to wake it up, but sometimes if I press the home or return button the screen lights up. Does this mean it has not gone into sleep mode and if so, is this a malfunction of the phone or a software problem? It's not happening all the time and a re-boot seems to solve it for a while. I have not made any modifications yet either.
Any ideas please. Thanks

 

[ElasticNinja]

do you use FB app?

 

[Silly Jilly]

No, I'm on payg dolphin and with their small data allowance I try and use wifi for most things.
I'm noticing now that sometimes any of the bottom buttons will activate the screen. I am right in thinking it should only be the power button on top that should do this yes?
Also, once or twice after unlocking the screen the camera is active, but it hadn't been used previously.
If it's not a known problem that I can fix, maybe I''ll walk it back to the shop and see what they say.
Thanks