remains, traces, footprint
蹟 centers on physical evidence left behind: footprints, traces, or remains that show where someone or something has been.
蹟 combines 足 (foot) with 責, which likely contributes the sound and the idea of something left behind or accumulated. The character is a variant form of 跡, which also means footprint or trace.
The left side is 足 (foot), and the right side 責 suggests something left behind. Imagine a foot leaving clear footprints in soft ground, each step a trace of where you've been.
For セキ, picture a detective examining footprints at a crime scene, saying 'I see key traces here': see-key -> セキ, and the footprints are the remains.