I think the part you are missing is that this was not an ordinary cellphone. These were pagers handed out by Hezbollah to the militants in their organization so they could communicate, specifically because they did not want to use ordinary cellphones out of fear of being tracked.
The only person who would be likely to have such a pager is a Hezbollah militant who is deemed responsible for secret Hezbollah information (i.e. mid-to-high ranking members). While it is technically possible that such a pager would get into the wrong hands, that would be the fault of the person who left his pager on the table or let his family play with it.