surreptitious

US /ˌsɝː.əpˈtɪʃ.əs/
UK /ˌsɝː.əpˈtɪʃ.əs/
1.

lén lút

done secretly, without anyone seeing or knowing:

:
She seemed to be listening to what I was saying, but I couldn't help noticing her surreptitious glances at the clock.