Beatrice and Benedick have a past, you see. When they do come together again after the battle, they engage in a battle of wits. At the end of which, Beatrice mutters "I know you of old." Futhermore, in the party scene, Beatrice will admit that she "lent her heart" to Benedick a while back. Both this things imply a romantic history between the two characters.
It is difficult to determine just how genuinely concerned she is about is welfare for there is some indication they had a relationship in the past and she remains angry and hurt by its outcome. That is not to say she wishes him ill, that she wishes he had come to harm in battle, for that is not the case. However, when she says "He is sooner caught than the pestilence" and then later in speaking to him "You always end with a jade's trick. I know you of old" (85, 143), she indicates they do have a past that left resentment in her heart.
Although Beatrice uses sarcasm in inquiring of Benedick's whereabouts, she is really concerned. She is concerned about his well-being, his success in battle, and those who are with him.