As I've said somewhere before, there isn't much difference in daily speech between this kind of sentences using "a" and not using the preposition. Commonly, the verbs that imply an idea of perception, like "mirar", "observar", "ver", "oír", "escuchar", etc., are followed by the preposition "a".
In daily speech, the usage is rather idiosyncratic, and it depends on what the speaker feels like saying.
Also, sometimes, "mirar" implies looking in some direction, so the verb is followed by "a", "hacia" or even "para".
- Mirábamos al horizonte. (We looked into the horizon.)
- Juan miró hacia la casa. (Juan looked at the house.)
- Cuando nos regañaron, mirábamos para el suelo. (When we were scolded, we were looking at the floor.)
My advice is don't worry, these sentences in your examples are synonymous in daily speech. If any one "corrects" what you're saying, they'll just be showing their personal preference rather than having a grammar principle to defend.
