Sometimes a senior developer will leave and it's not an indicator at all. EG: the founding CTO was more of a coder than a manager and as the company grows you

Sometimes a senior developer will leave and it's not an indicator at all. EG: the founding CTO was more of a coder than a manager and as the company grows you need a CTO who is more manager than coder - that is a tough bump. If the exit is clean/healthy with transition period and no hints of animosity then that's just normal events as a company grows and demonstrates resilience - yet still not comfortable. Bluntly: the CTO who takes you from $0 => $5M may not be the one who takes you from $5 => $50. Replace CTO with any other senior position. However if multiple senior people are leaving with short notice then you should ask them where they are going. Last three points above are 100%