Not sure if you've ever been in the military, but when I was there, if I had left a secure device or my gun somewhere out of sight/reach and someone else got to it, I'd get in a ton of trouble and probably go to prison.
Pagers employ unencrypted communications and (because they are receive-only devices) use a broadcast system to deliver messages to the pager. [0] Israel is publicly very, very friendly with at least one very wealthy Five Eyes country, and may have less-public support from many other wealthy and technologically sophisticated countries. If Israel happened to not have the domestically-developed capability to get a copy of every single page sent in an area of interest, they could ask their good buddies at the NSA, CIA, or other such global intelligence agencies to shunt that information to them in a timely manner.
Given the organization's sophistication, there is absolutely no way that Hezbollah believes that the contents of their pages are secret. The worst-case outcome of a lost pager is that the organization temporarily loses convenient contact to the person at the other end of that pager. While this could potentially be operationally disastrous, it's more like losing your service weapon than it is leaving the plans for D-Day on a public bus.