My guess is that it was relevant only at that particular moment. It was an expression of humans' attitude towards Clark. Just like criminals who are not accepted as 'normal' members of a society and stay behind bars in prisons or lines in this particular case, and treated differently from the rest of the society, Clark was also a stranger, an alien whose intentions or nature wasn't fully understood, so seeing 'behind the line' phrase behind Clark bears that symbolism in my opinion. So you can guess that he would be handed over to Zod.
Also note - if we continue our analogy with criminals - that he hasn't been tried/appeared before a judge yet. People see him later in action, and he is judged by his actions, and afterwards accepted as an ally i.e. acquitted.
And the reason it was painted over maybe because the rest of the phrase was irrelevant, and also if it said detainees - he is not really a detainee, wasn't detained, he surrendered. Or maybe because they couldn't really detain him, he ripped handcuffs off with ease.