You're probably on step 4 of her events, which has a random chance of her to appear in a maid uniform in the secret bedroom of Ryen. Use the Fast Travel system to the castle a few times and check each time if she is in the uniform now.
"Hellhound next step is unlocked only if Leneth gets pregnant. There is a 25% chance that she will appear in the Aldlyn Castle secret room every time you use the Fast Travel system. Talk to her to get her second CG scene."