Thanks for the last comment from Chard. To clarify with my experience:
- To avoid unnecessary runs: If you see a total of two lost sirens or wandering sirens, Serisaria will not spawn.
- If you see none or only one of those sirens, Serisaria MIGHT spawn, no gurantee, though.
If Serisaria does spawn, you will see the brief popup notice (A formidable opponent appears.). If you miss that, you have to go and look because Serisaria does not appear on track.
Best luck. Took me about 8 runs...