수고하세요 — what are you supposed to say back?

This one shows up everywhere: paying at a convenience store, leaving a café, the end of a phone call. Someone says 수고하세요 (roughly "keep up the good work") and… do you say it back? Just smile? Say thank you?

Share the moment it caught you off guard and what you ended up saying. If you've been here a while, when do you use it yourself, and when would it sound strange?