Although you say:
No, no. No non ethical reasons here (not really anyway)
You actually mean, "Yes, for non-ethical reasons" when you say:
The second reason is mostly being curious, at work. I wanted to know if any other staff were budging just browsing the net like I was.
Although these may be in violation of your companies policies and procedures you are definitely treading on unethical grounds here. This is the essence of ethical behavior, looking at something that might not be against the rules but think that "Hey, maybe I should not able or permitted to do this."
If you have a concern about you colleagues business related activities then you need to bring it up with your management. Of course, be ready to be critiqued yourself. Management can the take it up with their IT staff or, if you are the IT staff, they can authorize you to take action. In other words, as we always say here, "Always get written permission."
Of course, you will have to notify all of the personnel working at your business that they are being monitored. You will want to check into bannering for all of your systems and all of the methods of access. Then, once the banners are in place, you will need to train all of your personnel on the new policy and how if affects them before you can hold them responsible for adhering to it.
So, although looking at the information transiting the wire might be fun, is it worth your job or, possibly to likely, jail time?
Go forth and do good things,