catch sb napping

catch sb napping (to come upon someone who is unprepared; to surprise someone) — застигнуть врасплох

Also
catch sb off balance;
catch sb up short

be caught napping (to be unaware of imminent danger or trouble; fail to respond because of inattention; be off guard; to not be ready to deal with something at the time when it happens ) — зевать, прозевать, зазеваться; проворонить; быть застигнутым врасплох

Example 1: Hey, don't be caught napping! — Не зевай! Не прозевай! ("Touch all the Bases")

Example 2: The thieves caught the security guard napping. (thefreedictionary.com)

Example 3: The enemy soldiers caught our army napping. (thefreedictionary.com)

Example 4: The civil unrest caught the police napping. (thefreedictionary.com)

Example 5: Our competitor took our best customer while we were caught napping. (jolindaosborne.com)

Related vocabulary
[застать врасплох]

[asleep at the switch]
[catch sb off guard ]
[take sb aback]
[проворонить]
[catch sb with one's pants down]
catch sb on the wrong foot
catch sb off balance
catch sb up short