stay on one's toes(Featus); keep an eye open for(someone z484z); stay on alert(Artemie); have one's wits about oneself(Yeldar Azanbayev); keep one's eye on the ball(Азери); keep on one's toes(Please remember that we have no idea if this guy with the explosives will be watching us, so keep on your toes and be prepared for anything.4uzhoj); be on one's toes; keep nose to the ground(Himera); keep head on the swivel(Anglophile); keep one's eyes open(дабы избежать чего-л. нежелательного • "Just wanted to share a disturbing incident that happened around 6:30 PM near Lochdale Elementary today. While walking, three teenage boys on scooters came from behind and shot soft pellets or air beads at us using what seemed like a toy gun—for no reason. It happened so fast, we didn’t have time to react , and they later shot at a passing car as well." "Thanks. I will keep my eyes open for sure. I remember in high school a friend almost lost an eye to some idiots goofing off with pellet guns across the street." (Reddit)ART Vancouver)
