[she's still concentrating, but what was that saying? The tighter you try to hold on to something, the faster it slips away? Opening her eyes and looking at her tea.] I don't know. [there might have been a fleeting feeling of mischief, but did she imagine it?]
Other options:
no subject
[she's still concentrating, but what was that saying? The tighter you try to hold on to something, the faster it slips away?
Opening her eyes and looking at her tea.] I don't know.
[there might have been a fleeting feeling of mischief, but did she imagine it?]