I was contacted by a recruiter who forwarded my details to a hiring manager. That hiring manager contacted me and after some introductory chat asked me to solve a code challenge. I did the code challenge (about 4 hours of my time) which seemed to impress the hiring manager and the team.
Then I was phone screened by an engineer which went well. For some reason the hiring manager directed me to a different position so I got contacted by a different hiring manager. I had another phone screen with another engineer of this second hiring manager which went also pretty well. To a surprise I was directed back to the initial hiring manager for some reason and invited to an onsite. I attended the scheduled onsite which consisted of about 5 interviews in a row, most of them around 45 minutes tech and behavioral questions. I had the impression I did all interviews pretty well. Some whiteboard sketching was involved in one or two interviews.
Apart from the technical aspects which I solved fairly well, I also was very honest about my likes and dislikes on behavioral questions, also about potential reasons to leave my current employer. I suspect this openness may have raised some red flags or concerns for some interviewers and this might have influenced their decision to the contrary. It's perhaps down to cultural aspects that I'm not used to yet. Anyhow, I prefer honesty and directness instead of making something up to shine, maybe it wasn't the perfect approach to them.
In the end I wasn't extended an offer. Either because I appeared too direct on certain soft skills, or I'm too senior (expensive?) for the position they were hiring for (currently with a FANG company) or they just were kind and wanted to make my decision easier (sometimes interviewers might actually be of help rejecting someone they like for the reason that they aren't happy with their own future prospects at that company).
I'll never know the real reason, but do think they have a pretty good hiring process.