What are the most obvious telltale signs that he/she's cheating on you?

Hiding things doesn't necessarily mean cheating. I think u might be going bit far in that. I think that it has got something to do with that it wouldn't really mattered if you knew or not. By this I mean; nothing important. Yet I think your partner wants some things for himself and he doesn't want to give everything away. If he has no place where he can do whatever he wants on, he might end up frustrated. We often worry too fast before having real statements/facts and I think this is one of them. Your relationship has been going on for a while on and perhaps he also got bored of hearing the same question over and over again. (however you are kind and all, but if he has had stressed days it's very possible that he just not is in the mood anymore for further discussions with you, making the passcode something normal, as he can release everything in that without having to bother you (cause this could also be a reason, not wanting to bore you with his problems) I don't assume your man is cheating on you, I think it's normal that everyone has their own special place, to do whatever he wants. So don't waste braincells onto it

That's what got me thinking in the first place cause he never hides anything from me before, so i was mildly, no, make that very, shocked to find out about the passcode.

I did observe that he easily gets angry when i confront him about it. i know it's his phone, but he never put a passcode on it before as i never look into his cellphone a lot just to play some games in it or read mobile ebooks on it as mine doesn't have those features. and when i asked him about the passcode, he just shrugged and didn't offer much as explanation. that got me worried.

Frustrating isn't it? In my case, it went further than him changing his email password too. Although, after a huge fight (in which he told me he wanted to reconnect with her because she was "special"), he said nothing was going to happen - he just wanted to know about her life and what had happened to her Then, he went to visit some family and, while he was there, he IM'd the girl via Facebook and had a conversation that way because he figured I wouldn't know. Believe me, I always found out the things he lied about. He also admitted that he tried to get her to agree to meet him in person. She never agreed but it was clearly what he wanted. I think the driving factor was that he was hoping she thought of him as "the one that got away" or that she's thought of him "after all these years" - which, clearly, she does not. So, again, I'm not saying to accuse your guy at every turn but trust your instincts. If I'd learned to trust mine, I'd have gone through a LOT less heartache.

Yes thinkingoutloud, I had free access to his phone and he never stops me from using it as he knows i'm not one to read his emails or sms messages. I just use it to play games and read ebooks. Now that you've mentioned it he could be communicating with an old flame as i heard recently that his "first love" just broke up with her boyfriend. Could it be he's comforting her or something deeper? :(

Thanks funorb12, i guess that's finally what got me bugging. Not talking to me when he gets home from work did get me worried cause we never did miss a day that went by without talking. It was our stress-free time. And now I'm really stressed-out cause what, he just decides to break a four-year habit, suddenly? And with no explanation. And when he put the passcode, I was really at wit's end. how do I confront him without making it seem like I'm accusing him?