search, seek, investigate
搜 centers on actively looking for something: searching, seeking, or investigating to find what is hidden or unknown.
搜 combines 扌 (hand) with 叟, which likely contributes the sound and relates to searching or seeking by hand. The exact historical development of 叟 is uncertain.
The hand radical 扌 reaches out beside 叟, like a hand probing through a cluttered room to search for something hidden.
For ソウ, imagine searching through a messy room and finally finding what you need, letting out a satisfied 'So!' — So -> ソウ, as your hand 扌 completes the search.