Exodus 3:1 goes,
וּמֹשֶׁ֗ה הָיָ֥ה רֹעֶ֛ה אֶת־צֹ֛אן יִתְר֥וֹ חֹתְנ֖וֹ כֹּהֵ֣ן מִדְיָ֑ן וַיִּנְהַ֤ג אֶת־הַצֹּאן֙ אַחַ֣ר הַמִּדְבָּ֔ר וַיָּבֹ֛א אֶל־הַ֥ר הָאֱלֹהִ֖ים חֹרֵֽבָה׃
And Moses hath been feeding the flock of Jethro his father-in-law, priest of Midian, and he leadeth the flock behind the wilderness, and cometh in unto the mount of God, to Horeb; (YLT)
I think that the relevant phrase here is:
וַיִּנְהַ֤ג אֶת־הַצֹּאן֙ אַחַ֣ר הַמִּדְבָּ֔ר
The word אַחַ֣ר, to my knowledge, usually does mean "after" or "behind". The following word, הַמִּדְבָּ֔ר, means "the wilderness". So translating this phrase as "behind the wilderness" indeed makes sense as a word-by-word translation, but I'm a little unsure what this actually means.
Does it mean that Moses led the flock through the wilderness? Or does it mean that the wilderness was already behind them? I'm trying to visualize it, you see, and the two scenarios are a bit different when visualized.