Is this a bug on facebook?

[Rares]

I have 3 devices: an iphone, ipad and a laptop and I check my FB ACCOUNT everyday, sometimes on my laptop or my iphone, sometimes on my ipad BUT I always make sure to sign out: on iphone and ipad I always close the app and kill it in the multitasking tray and on my laptop with Chrome I click sign out after I'm done but my friends say that I spend my time on facebook a lot, a coligue in my class checks his FB all the time and when he looks at my FB, I am always online even though my laptop is home sitting there password protected and shut down and the FB app as always killed on my iphone. Why am I online when I am actually not? I also changed my password and it's just a bunch of random words and numbers.