under the radar

(idiom) doing something without other people noticing

Example Sentences:
A: I heard you had a new girlfriend!
B: Yeah! But she’s kind of young. She’s only 15.
A: Wow! What does her father think?
B: Well, we’re keeping our relationship under the radar because we don’t want him to know.

The employee didn’t want his boss to find out that he was looking for another job, so he did all his searching under the radar.

From the News:

Wings flying under the radar

Media:

Similar idioms:

1. clean break
2. eat humble pie
3. tie someone down
4. take a dig at someone
5. pass something with flying colours