If you're going to be away for a few days, tell your room mates.
One of my friends has this problem with one of her house mates. I've met her a few times and she seems okay, like she'd be okay to live with, but she just doesn't see it necessary to tell anyone if she won't be home over night - she'll just not come home one evening after classes, then be back a couple of days later without any warning. The first time she didn't turn up, the rest of her house mates were going frantic - they thought maybe she'd been in an accident or something, given it was well past her usual time to be home.
They've just had to learn to put up with it because she doesn't see why she should tell anyone about her weekend plans or whatever, and she's right, she doesn't have to, all she has to say it 'I'm not going to be here thursday night, I'll be back on friday' so her HMs don't worry about her.
Please, just let someone know where you're going to be and for how long; one so they don't worry unnecessarily, and two, in case an emergency comes up (for example the house flooding, unlikely but you never know), somebody knows where you're supposed to be so they can get hold of you more easily.
Also, if you're already away and you plan on staying an extra night or two, PHONE - my brother does this all the time to my mum (and I'll admit, I used to do it as well until I figured just how much it made my mum worry). It's not that hard just to pick a phone and tell someone you'll be home a day later - not doing so just makes people worry, plus it's pretty rude if someone is expecting you for dinner or the like on the night you were originally due home.