"Alert eyes" means that someone's eyes are wide open and attentive, showing that they are fully awake and paying close attention to what is happening around them. Full definition
They make sure baby is wide awake and relaxed with alert eyes before trying to connect. (gottman.com)
Dogs often turn their heads away from a threat to avoid conflict, and this boxer is doing so while also maintaining alert eye contact just in case the threat becomes more imminent. (mnn.com)
It has long ears with close curls, hazel - brown, alert eyes set well apart, and a sufficiently wide nose to ensure excellent scenting ability. (puppiesndogs.com)