Hela knew Hewitt's attitude towards her going out.
As long as there was no danger, Hewitt wouldn't interfere with whatever she did. If she encountered any truly lethal danger, the necklace called Heart of Sanctuary would create an absolutely safe space and send her location signal to Hewitt.
Other than that, there were items similar to the Undying Talisman and rings that allowed her to hide in the Time Gap, which could protect her safety.
However, these tools would not activate if she didn't encounter real danger, because they judged whether Hela was in danger by directly examining if she would suffer psychological and physical harm beyond a certain extent.
What they detected were impending events, the future of the next second, and fate itself.
So, if Hela didn't meet someone who really wanted to kill her and had the ability to do so, the Heart of Sanctuary wouldn't work, even if she was locked in a cell.
Because it wasn't necessary.
