In Greek, the word "sheep" is not used in this passage: it is added in Biblical translation. Jesus seems to have avoided it intentionally. The Greek has just the pronouns and number. This makes the verse seem less about sheep and more about anything or anyone lost and discovered. The article ("the") used to identify "the ninety-nine" can be either masculine, referring to people, or neuter, referring to sheep.
As we saw in the previous verse, Matthew 18:12, the meaning of a verb changed from "becomes" to "belongs to" because of the following word. Here, the same verb has a third-meaning, "happens," which is its meaning when applied to events. Here the event, "to find" follows the verb so again in changes meaning. This phrase, "happens to find it" is casually humorous but it is not translated that way.
The negative used to describe those that head not wander is not the negative of facts, but the negative of opinion and desire, the sense that the ninety-nine hadn't wanted to wander. This explains why the shepherd could leave them alone upon the mountains. This changes the wandering into a decision not just an action.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.